Toy Hauler Frames

What is the typical main beam size on a 30-35ft bumper pull toy hauler frame? I'm converting an old travel trailer and just want to be sure the frame is strong enough. I'm a structural engineer so I've run some calculations and I have a beam sized up. But since the structures I'm used to designing don't move (if done right...LOL) I just want to make sure I'm on the right track. And before anyone asks, yes, I'm aware of axle placements and tongue weights, etc. That's actually one reason the frame will need beefed up since I'll be moving the axles back a bit, lengthening the span from the wheels to the hitch.

Thanks!
Bob E.