I'd probably go the plywood route as well. Might consider coating the plywood in a fiberglass resin, water sealing, or undercoating before installing. I'm thinking I'd probably undercoat or paint it first, then put a coin pattern rubber mat (like the shop floor stuff) in the bay. Our's has minimal angle iron supports under that plastic. With the spare tire in there, you can see the angle iron flexing down a bit. Thought about beefing this up a little, but that's on one of those round toits.
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.