Most class C's have 3-4 seatbelt locations behind the drivers compartment.
However it is only a few class C manufactures that use steel cage around the house area, and fewer still that have any sort of roll over protection. Not that you 'plan' on driving in situations where the RV will roll on it's side or go off road with it, or take corners to quickly. . .
Yes it would be nice if a few more manufactures would build on the F-550 crewcab and chassis, with 4 wheel drive options. Thor does build on the F-550 and normally includes a diesel option, also can include 4X4 too! Yes it is a $136,000 coach. They had one at the Fairview Camping World a couple of months ago for sale. I looked at it, but the coach curb weight was really high. Not up to the 19,500 GVWR, but still well over 18,000 pounds! So not enough cargo rating for me. . . Had they included 4X4 package, that would have pushed it even higher, and you could not safely carry 1 passenger without going over the 19,500 GVWR.
Good luck on your RV search! If you want a F-550 based class C, they are out there. I think that there are even a couple of class C toyhaulers built on the F-550 chassis.
Fred.