Do you know what they can tow?
Guess most are based on 350 or 450 chassis, but they already have lots of weight with the living quarters......
Most are rated for 3500 to 4500 lbs. ONLY, pleeeeezz be careful, you just do not know how many bent frames I've seen from over loaded towing, and you would not believe some of the things I've seen towed.
With that said, you can modify frames for heavier trailors, but you have to watch your GCWR (gross combined weight rating)
Most RV manufacturers lengthen the frame rails, some to Ford's specs, some not.
Get under the coach and look at the frame rail and the hitch. The extention should be both welded and BOLTED, as Ford supplies the actual holes in their frame for the extentions.
Look at a new National RV class A extention, it will scare you to death, They are a very good coach over all, but the way they extent the frame is appalling.
David