[I apologize for the techie nature of this post, but the good stuff is right at the top, and the preachy stuff is at the bottom.]
My wife and I recently made our annual pilgrimage from our winter haunt in Arizona to our summer place in Maine. As we started out, we weighed the rig on a CAT scale. As usual, the CAT scale took only 10 minutes and $10, and gave the following results:
Position.....CAT wght....Rated wght (sticker on driver's door)
Front axle.....3400.........3900
Rear axle......3940.........3850
TM axle........3360.........3500 (axle limit) - 4760@60 psi (tire limit)
Total wght..10,700 pounds (this is the gross combined weight of the rig)
Weighing Conditions:
TM 2720SL
No black, gray, or fresh water
6 gallons of water in the water heater
About 10 pounds of propane in one tank, other tank empty.
Two people in the front seat of the F-150 tow vehicle.
Lots of stuff in the F-150 bed, including a 300-pound table saw (don't ask).
WDH cranked up to our normal travel position.
F-150 tires - Goodyear Wrangler SR-A P275/65R18
Sidewall says max load = 2601 pounds at max pressure 44 psi cold
Actual cold pressure about 41 psi
TM tires - Goodyear Marathon ST225/75R15
Max load = 2540 pounds at max pressure 65 psi cold
Actual cold pressure about 60 psi
F-150 ratings taken from driver's door sticker
So it looks like everything was OK for our rig. The F-150's rear axle was a bit overloaded - I should have moved 100 pounds of stuff - but nothing else was over its rating, and the 15-inch tires on the TM had lots of margin. But I noticed one thing. The TM had 3360 pounds on its axle. Since the WDH was engaged, the TM actually weighed more than 3360. A bunch of calculations and techie stuff [deleted for your reading pleasure] confirm previous conclusions by other members that the total (gross) weight of my TM was around 3800-4000 pounds. This is why I get nervous about towing a TM with a tow vehicle rated at 3500 pounds - especially when you remember that the tow vehicle's tow rating is decreased by the weight of everything you put in it, including cargo and passengers. If you put 400 pounds of people and cargo in your tow vehicle, the 3500-pound rating becomes 3100 pounds. Towing a 3800-pound trailer doesn't seem like a good thing, even if you "never had a problem doing it". And even worse if you tow with water in your tanks.
It also suggests that without a WDH, the hitch weight of my TM would be on the order of 560 pounds, which exceeds the 500-pound rating of most weight-carrying hitches. So again I get nervous about not using a WDH unless your hitch rating is at least 600 pounds.
Bill