Being a 2000, does it have the radiator trans cooler or just the aux trans cooler in front?
As mentioned, definitely upgrade the aux trans cooler to the 1/2" tube 5R110 cooler or the Dorman 918-216, they bolt in.
If there is no radiator tank trans cooler, maybe add a second cooler like the B&M Super cooler 70274 next to the 5R110 cooler.
Hellwig anti-roll bars, front and rear, really help keep the Ex under control in winds and winding roads.
Checkout the Ford OEM finned aluminum diff cover that was used starting in '08 or the Dorman 697-725 version.

 

Mine took 9000 lbs (4 cubic yards of wet compost from the local farm supply store in a rented dump trailer) with very little problem at highway speeds the first time around...

But then putting a 6000 lb 2012 Expedition EL on a 1500 lb Aluminum Trailer (Total 7500 lbs) for 30 miles on the highway apparently heated up the transmission enough that it puked several quarts of ATF on the ground.

2003 4x4 Limited; V10; 3.73 Axles; 2014 wheels wrapped in 275/65R20 Load Range E tires (35 inch diameter)

 

That should not have happened....

 

I'm aware.

That's what I get for relying on a 20 year old vehicle as a workhorse.

 

I have a 21 year old workhorse, that still should not have happened.
Run a flow test on your trans cooler lines, maybe there is a restriction or the bypass spring is weak.
Do you have the OEM aux cooler, what were the engine coolant temps?

 

I didn't have the laptop on the seat. Stock trans temp gauge and coolant temp gauge didn't move past their happy middle area where they sit normally.

I do believe I have an Aux trans cooler. It's the OEM one. I had been meaning to put the 6.0 one in.

 

I have a 2000 Ex w/V10 and have done EXACTLY what you are trying to do, and then some. My 36ft TT and Ex came in at a combine 16200 ( I travel for work) and live in Montana at 6k elevation with nothing but mountain ranges in all directions. I was disappointed in towing what was supposedly within specs for this rig from the first day I got it 5yrs ago. I've done nothing but mods over the years, and while slowly making improvements, its nothing like a modern truck. I started removing the PO's 8in lift and 35in tires and went with SD springs/4in lift/32in tires. Then I went with 5Star tuning and that was the single most effective mod I've done.....and also the cheapest ($500-ish). It mostly helps out the shift strategies in the trans over the stock programming, but also gives a minimal, almost undetectable bump in power. Next, I swapped out the stock 3.73 gears for 4.30 gearing for about $1300 per axle. Again, that helped helped a bit but not as much as I expected and obviously rev'd higher/burned more fuel loaded or unloaded at any speed. After that, the only other worthwhile investment was exhaust ($950). I have a fully-custom dual exhaust setup from the manifolds back. Didn't go with headers because they are way overpriced for what little gain you get, so I stuck with stock manifolds, a better/bigger y-pipe, a new high-flow cat dumping into a dual exhaust setup behind each rear wheel. The exhaust definitely helped it to breath better and I could maintain speed better up a grade while pulling the heavy trailer. But, it still wasn't good enough.

When pulling heavy, I only tow at 65mph. Faster than that and I'm just burning up too much fuel. The wind is the biggest issue I face in my area (Montana, Idaho, Utah) but the passes are no joke here, too. Its not really the weight, but the wind resistance. I've pulled SUVs on trailers at 70-75 with no issues uphill or down and the mod'd V10 wouldn't break a sweat.....but that is a much more aerodynamic setup than my TT. So what is a fella to do? Well, I bought a newer truck. Don't need all the seat belts anymore since the kids are mostly grown and out of the house now. Also don't have the pets to take along since they have had to be put down. I wanted something more comfortable, more economical, and maybe a little more modern. So what did I get? No joke, a 2011 F150 with the 3.5 EB. Other than seating 8 people, it does everything better (IMO). Yes, I'll need to put air bags on it, but that is nothing compared to all the other work I've done to the Ex. I've only had it for a month, but have already towed my 18ft jet boat across the state, towed a 12ft flatbed loaded with furniture across 3 states, and hauled my 36ft TT across the state. I get 18-20mpg unloaded, 13-15mpg towing lightly, and 8mpg towing heavy against the wind. The Ex could never get this kind of economy under the same conditions for the last 5yrs. I can easily pull heavy against the wind @ 70mph if wanted, but I still keep it around 65mph. Even though mine was a 2000, I never had issues with engine or trans temps when I switched to a 170 t-stat. I monitored all daily driving with a ScanGauge II. Sorry for the small novel, but thought you might like to hear my experiences with a similar scenario.

 

@inthedirt - How's the 2011 F-150 handle the 36 footer TT?

I'm looking at going to that 2012 Expedition EL, as I DO need the eating for eight... (I've got four teenage nieces that I take on vacation during the summer along with my own two kiddos.)

For my testing purposes, I'm going to be borrowing my neighbor's 20 footer flatbed equipment trailer and loading my Excursion on it for a lap of the Washington DC beltway. Should be about 9000 lbs and the Expy is rated to do 9200.

I'm about 20 miles south of the Beltway and the beltway itself is a 60 mile loop, so 100 miles round trip.

I've picked up a Blue Ox 10k lb weight distributing hitch; something I've never had to use with the Excursion and will be transferring my Prodigy P3 brake controller to the Expy for the test.

 

If that Expy has IRS, get ready for some squat even with a WDH.

 

Our 2003 Expy was the first year of the IRS, I think they all are since then, and yes, it squats.
Our 2002 Excur would squat until I installed new rear leafs, rides fine now.

 

Agreed; I've got stock springs on the Excursion. You can see the squat in the Expedition towing pic.

I didn't have much room on my 16 foot aluminum trailer to roll the Expedition backwards and adjust tongue weight.
When I borrow the 20 foot trailer, I can roll the excursion back and forth to play with tongue weight.

 

I have confirmed that there is indeed a transmission cooler between the A/C Condenser and the Radiator.

 

Did your ATF leak have anything to do with reversing with the loaded trailer? That’s a fairly common event with the 4R100, I use 2 wheel drive low range when reversing our heavy travel trailer to help avoid the input seal leak, so far it’s been good.

 

I purchased the OEM 6.0 transmission cooler and it is huge compared to my 2000 6.8’s. It is almost the size of the radiator.
I also installed a spin-on remote filter to help keep the fluid pristine.
Autometer transmission gauge is used to monitor the heat. It also has a warning light I can set for what I think is time to pull over and cool off.
Just a suggestion or two.