FWIW, I have a 96 Fleetwood Southwind, 37' tag axle on a GM chassis.
I also have the 3500 lb. tow rating.
To get additional length, the chassis was extended by welding beams onto the original framework.
It is my understanding that can be a weak spot. If excess weight is towed it could possibly break the welds.
If I was going to tow something 4000 pounds a few miles at a slow speen, I would probably just do it.
But, towing a 'overweight' toad on a normal or extended trip. I think it's just asking for problems.
I'm definitely not an expert on towing or chassis construction.
Just giving you my opinion.
Enjoy your travels.