Paws,
Towing attributes is all about the 'whole system', not any one component...though
one component might be the weak link to dictate to the whole as what it should tow
There is a world of difference between 'can do it', to 'do it well', to 'should', etc
Do NOT use 'dry' weights, as that is a newbie mistake....unless you do plan
to tow it 'dry'. Meaning...nothing is added over what it came from the factory
with. Including none of the dealer installed stuff.
Go out and weigh the setup, axle by axle, fully loaded. That is the only way to
know where you are in reference to the ratings
P class tires don't help (passenger class and must be de-rated a min of 9% when used on trucks)
Am not a fan of IRS and note that it is okay when new, but over time, the precision
pivot joints become a potential issue. Assume your year Expedition has IRS
The lever arms (moments in eng speak) work against any short wheel base TV and
good WD Hitch systems do help. It is still there...just muted by WD
Then gather the Expeditions ratings: GVWR, F/R GAWR, GCWR and do the simple math
from your actual weight numbers. My guess is you will find that you need a bigger TV
Here is the formula:
GCWR => TV + Trailer + hitch/stuff/etc