I know this is a FR forum, but the only one I can think of right now with the beefier frames for this is Northwood, makers of the Artic Fox and others. As they refer to them, "HD certified structural steel offroad chassis". They're one of the few that build their own frames. I've always liked Northwood, they've just never had a floor plan that suited us, and you don't see them sold around where we're at. Mainly north west I believe.
John and Rebecca Dickson
Emma-7 / Little John-5 / Iva-1
Full Timing Again, Rev B
2013 Ford F-350 Lariat CC LB PSD
2015 SOB TT - With OC's Awning Poles (#8)
At least in Heaven, RVs will be perfect, and I won't have to keep fixing them.