Have a 2000 Excursion 6.8l v10 4x4. Recently bought a TT, before fully understanding payload. Was only paying attention to tow capacity.

Went to a scale to get proper weights, full tank (44gal) gas, 4 kids, hubby, myself and the dog. With the trailer loaded and hitched up.
Got 4 weights at the scale, front axle 4,100#, rear axle 5,360#, third axle/trailer 6,000#, total weight 15,460#. From what I've read, my Excursion has a GVWR of 8,900#. Which puts us 560 over payload weight..? So I gotta get rid of my kids? Or 3 kids and the dog? It looks like we are good on the axles GAWR and have plenty of weight to spare in the trailer on the towing end. Our tires are E rated at 3,415#. So we are good on the tires. But that dang payload inside the excursion is ridiculous! Shoot, even without the trailer hitched up, if all 8 seats are filled we would be right at max payload. This doesn't seem right. Is my math wrong??
We have a brake controller and WD hitch, we aren't sagging at all. Going to go to Les Schwab this week to habe tires rotated, inspected and set to max psi before we go camping this weekend. But I am worried about my payload math.

Also, we overheated on a mountain pass when we bought the trailer and were bring it home. Hubby forgot to take it out of OD, and we were also low on coolant, and in the middle of heatwave here in Washington. When we got home hubby put it in a new radiator and hoses, and I put a sticker on the dash with a reminder to turn OD off lol. It feels really good when towing otherwise.

Other than that.. Any tips, advice, clarification on payload..?



Excursion specs Trailer specs

 

Yeah, you have it right as far as the limited payload on the big EX.
Federal DOT regs look at the axle and tire loading ratings as the limit and not so much the manufacturers payload info. In my opinion if you are under on your axle and tire weight ratings you are good to go as long as the WD system is properly setup.
The 4R100 transmission was designed to tow in OD, leaving it in OD will not cause any overheating, it will downshift when needed like its programed to do. Turning OD off is a good idea only if its constantly hunting back and forth between OD and third gear as that will build heat, heat is the enemy of long trans life. Do you have some way of monitoring the trans temp (besides the dash gauge) while towing? Its a good idea to keep an eye on the trans temp during a tow and if it gets up over 230-ish pull over and let it idle in Park to cool down, never shut down a hot trans as it will just sit and cook. It sounds like your overheating adventure was due to the low coolant and the extra load of the TT.
Using OD and being able to be in OD on the flats will also increase the towing fuel mileage a bit, going from 7 to 8 MPG is a 14% increase which is a pretty good bump.
Your EX's tires don't need to be at the max pressure, that's only needed to carry the tires max payload of 3415lbs X 2 = 6830lbs, you only have 5360 on the rear axle so check the tire manufacturers inflation weight tables to see the pressure required for that weight. The front axle weighs even less, so it will require an even lower pressure to carry that weight. I typically run about 5 PSI over what the tables call for for my scaled axle weights. The ST tires on the TT should be run at their max listed sidewall pressure, those ST tires are a special case and extra attention needs to be paid to them especially if they are any of the cheap Chinese brands. Some have a 65 MPH speed limit and they all age out quickly, so check the build dates and plan to replace at 3 years of age (if higher quality ST are being used I would feel safe stretching the age out a little).
Your "?" for the TT's cargo weight is 2075lbs as it is simply the GVWR - dry weight. Most TTs never actually see their dry weights as that sometimes doesn't include the battery or propane tanks plus all of the standard stuff like power cord and sewer hoses that are always onboard.
Our V-10 4X4 EX is our dedicated tow rig for our 12K lbs TT, we have pulled all over the Eastern states for the past 8 years logging over 30K miles with that trailer and another 15K miles with the prior 9.5K TH.

 

We only have the dash gauge to monitor temp right now. I can look into an additional temp gauge.

Is there a problem with filling tires to max pmi? Is better to have them filled more, than under filled?

 

It will ride firmer than necessary.