I'll be doing just that in the next week or weather permitting.
Using 2/0 wire and industrial battery connectors.

High idle mod would help.

Something I found out with my trailer; it is equipped with hydraulic landing gear.
With the truck connected and battery not installed in the trailer the truck did not provide enough current to operate the jacks.

 

Wouldn't a small generator be a better option?? Solar panel???

 

What I do is put the 3000W Honda generator in the bed of the truck, using a 4" PVC pipe running underneath the camper, I run an extension cord (30A RV type) to the back and plug in my camper as I ride down the road. I can even stop an hour before I arrive at my destination and run the A/C to cool it down while I drive.

This is easy. A 3000W or smaller Honda generator runs verrry quiet and sips gas, gives you more versatility.

 

You have to also remember that lead-acid batteries charge best when the amps-in are in sort of a bell shaped curve. You have to taper the amount of amps going in ramping it up in the start then charging at the rate the batteries can handle without over heating then slowly reducing the amps in at the end to get a full charge. I know this from the program my solar power system uses to charge my lead acid battery bank. Most car battery chargers do the same thing. So be careful just throwing more amps at the batteries.

 

ZXKeller and Porthole,
I agree that high idle will help, but even with that there is no way the stock 12 gauge wire can carry any amperage the 20-30 feet it needs to travel from alternator through ford ftrame to ford camper plug, through the plug connection and camper wring to the camper batteries. 12 gauge wire can carry 10 amps only 19 feet, 20 amps only 10 feet, and 35 amps only 4 feet. So stock wiring is only good for a slow trickle charge, which is fine if you are driving 5 hours to a campsite and using only minimal camper lights or pluggin in for a weekend.

6 gauge wire would carry 35 amps 22 feet; 20 amps over 30 feet.
4 gauge could carry 50 amps 30 feet.

Given that the ford already has 2 very large batteries, that 12 gauge wire is too small for the ford regulator to see the camper load in any serious way, even if 2 camper batterise are very low. 2/0 wire may be tougher to handle but may not be as overkill as some think- it would certainly let the ford's regulator "see" that there is a need for more juice back there and deliver it. This is particularly true for the dual alternator setup. It is my understanding that I runs one, the other, or both depending upon the load detected.

That 2/0 would allow more much juice than Ford's std trailer plug/jack can handle, but only if you had 2/0 on the camper side too which is not the case. My ford-jack to camper "charge" wiring is also 12 gauge, so this would leave me

Porthole,
please update on results of your effort and explain how and what you use to make the connection(s) from truck to camper for both positive and ground, ad whether you will use this when travelling or just when parked. Will you need a 2/0 ground too or do you think the juice travels back through the hitch?

 

These are the connectors I use
McMaster-Carr

I use the 175 amp series, not because of the capacity but because of the physical size.
The 350 amp version is just too big.
I use 2/0 welding cable. Welding cable is very fine stranded wire and very flexible.
The 2/0 welding cable is more flexible then 14/3 Romex.
I use the 2/0 cable/wire ends
I use a handle on both my jumper cables and trailer end to help disconnect.

On my GMC I ran only one wire and used a 3’ jumper to the frame.
That didn’t work so I ended up adding the ground all the way to the engine block battery ground.
I will be doing the same on the SD.
I haven’t looked at exactly where I will make the connections yet, but if the starter is available that is where I will pick up the B+.

Will be using a marine grade fuse block and I keep 150 to 250 amp fuses in the tool box.
Because this truck is so long I will probably be adding a second bumper connector on the front end.

My trailer has a 5.5 genny, so it is not a real issue with that – yet. On the list of things to do though is to add a 3K inverter, and this will not only be fed by house batteries but connected to the truck with 2/0.

I’m hoping to be able to not only use the TV while traveling (my wife will lay down in the trailer on long trips), but to be able to run the AC prior to camp arrival.

And for those days where “no generators allowed”. The super quiet truck should be able to supplement the house bank for some time.

My motorcycle trailer already has the hookup, so I’ll snap some pics of that. The main difference between that and the toy hauler is that I will use battery connectors on both the truck and toy hauler. Don’t need the big wire all the time.

McMaster-Carr is also where I get the 2/0 wire and heavy duty jumper cables.
McMaster-Carr

I bought the 20’ version, in retrospect I should have gotten the 25 footer.

 

A lot of guys out here just run the Gen in the hauler going down the road. I've got a buddy with 2500 hrs still going strong

 

I installed a 2000 watt inverter in a tongue mounted box to feed 120 volt to multiple toy chargers, lights, and a battery charger for two deep cycle batteries. I ran a 1/0 cable to the rear bumper where it terminates with a small forklift style connector. This provides power to the inverter to charge batteries and provide 120 volt. I also have a pigtail wired in for a small generator.

 

After reading all this, if you do a lot of dry camping sounds like the best route is a generator - although not the cheapest. If you are close to having enough battery power as it is and just need a little more time, maybe adding a battery or 2 to the trailer will help. Just make sure you charge it up before you leave home and as mentioned, it will continue to charge while driving.

 

Charging off of the truck no matter what will not charge the batteries fully, if you have an inverter, use the truck batteries to power up the inverter and then just plug your trailers 120 v into it, this will use the charger in the trailer which will give you a full charge. Also change your 2 12 volt batteries for 2 6 volt ones and run them in seires to make one large 12 volt that will last twice as long as 2 12 volts will.
I have a 3000 watt inverter that is mounted in my trailer and I connect it to my truck batteries will a couple of 1/0 cables with quick connectors, the small welding type of connectors. I have the female ends of the cables mounted behind the front bumper.
I already had the inverter, but I am also looking at a gen too since we do lots of dry camping.

 

First pic is my motorcycle trailer. The bike trailer has a 2KW inverter installed.
I'll be using the same setup on the 5th wheel to power an inverter and to have more current available at the trailer.
I have already run into a minor problem where I needed to activate the hydraulic jacks on the 5th wheel with the battery removed. The trucks trailer connector harness could not supply enough current.

2nd pic is a set of 20' heavy duty jumper cables cut and the heavy duty battery connectors installed.

 

Nice setup Porthole! I had to do the same.

 

Porthole,

Nice setup and thanks for the info. Those cables will take all of the Ford's alternator power all the way to the camper or boat battery when needed, and be suffecient to run those front hoists or hydraulics without camper batteries; and be out of the way othewise.

Large 12 volt cables to just shy of the the 7-way and standard 7-way and camper wiring beyond would help a lot because you get "full" power at least at the rear bumper cuttingoff 20 feet of run, but yours eliminates the concern oringinally expressed.

Yes a small solar charger with good regulator is a still a good idea if you leave the camper batteries unattended for weeks at a time.