Keep your eyes on the battery box under the stairs. . The floor gets soft after time with 250# of batteries in there. No fun if they fall out. 1/2 in ply seems to work .
Check your water tank. It also falls out the bottom, cutting many air and electric lines.
For this, I ran square tuning across the frame rails and set some 3/4 ply on the tubing , under the tank . Removing the load from the body and placing it on the frame. Gravity always wins .
Our battery box started coming apart where the side meets the bottom of the box, on the side towards the rear of the coach. It is the reason I switched the start battery to a 1000CCA truck battery three years ago. Kept a 8D for the coach battery.
This summer, due to the covid-19 we stayed fairly close to home, so I had time to reinforce that side with some 1-1/2" angle, both inside and out, along with nuts and bolts. Couldn't get over how thick the fiberglass was. It is only on the surface. There is an inner material of something.
Our water tank seems solid. Of course the coach only has 85,000 miles on it. I make sure I never fill over 3/4 because I think there is a crack up around the overflow flange that will let water flow down inside the "firewall" between the tank and the wall.