I just bought a Trail Lite B+ 211s with 8.1 Chevy engine. This is my fifth motorhome (others range from an old 20' Vagabond, and three Winnegagos, from 19 to 32'. This vehicle is fantastic. I am...
You could just use some 1x2 (or even 2x2) 'ribs', glued and screwed to the underside of the plywood. This will help prevent sagging. 2 or 3 ribs should be plenty. I'd probably go with 3, evenly spaced.