replace coroplast with steel/aluminum

There is the problem right there -- in Calif, the paint is all like weak pastel watercolors, because really good oil paint creates volatile compounds that cause smog. This is very annoying; but on the other hand, the smog is WAY better nowadays than it used to be. So we reluctantly put up with all of the regulations.

Yes the plywood will get wet while driving. But with two coats of oil based paint on both sides it will last for many many years. You must use fir plywood. No birch. And no latex paint.

PR, wouldn't the plywood get wet? Or does the paint protect it from moisture?

My old man did the whole underneath of his fifth wheel. It to had the Coroplast. First was to remove all the coroplast. Next wears to clean up the shoddy workmanship that he found. The then added 1.5x1.5x1/8 angle iron every four feet from fram rail to frame rail. He used 1/2” fir plywood that he painted both sides. Plywood was secured with self tapping wafer screws. Any areas that he could not get tight (1/2” or less) he sealed up with silicone.

This all came about after getting a rodent in the 5’er. After it was complete he went 5 years with out an issue. Then sold the 5’er for a truck camper :)

Back many years ago , we used to get cast off aluminum sheets from the newspaper printing process to use for patching up salt destroyed bodywork. It was free and bent easily. I was going to try to use some to do underbody covering to reduce air resistance, but never got around to it. Results probably would have been inconsequential, but when your in your teen, sometimes these projects seem cool.
I don't know if the newspapers still use aluminum sheet for their print process, but it might be worth looking into.

Great idea about the wood frame -- otherwise, I would just be putting armor over the plastic, which is weak stuff.

Dan, I built new wheel wells for my old Starcraft. The originals were destroyed by flats and spinning belts from a previous owner. I threw out the plastic remaining, built a wood frame above the wheels, put aluminum panel in the corners, and installed 404 steel sheets, glued and screwed on to the wheel well frame. 404 is what they use for garbage truck compactors and Humvee armor. I feel a lot more secure with that above my tires.

Great idea! Thanks.

Wheel cans is a broad description but ... a three sided can, if you will, that protects the front, inside from the tires, and bottom of vehicles.
Wheel wells made of sturdy stuff like thick aluminum or steel.
On Towables tires are generally not going to grenade or explode as bad as when it happens on a MH or semi.
We have had many many postings over the years from wrecks, and destroyed structures of MHs and Towables when tires go bang.
Working them into an existing RV seems to me, is harmonizing with the tire opening in the skirt or body asthetically would be the biggest hurdle but maybe not. Even if it destroyed the flimsy plastic skirting, on a towable, if the underside of the RV, and exhaust on mhs, and wiring and plumbing and tanks were protected I think a lot of expense and some injuries and deaths could be avoided.
I toyed with the idea when we had a Fifth Wheel, even drew up plans and made, cardboard templates....and then we traded for our MH.
It is an easy to envison concept. Two axles two tires per side, something along the pattern of fenders sold for trailers and of course a slot or arrangement to let the axles move but still shield everything else, mounted to Towable body if strong enough but leave plenty of vertical travel for axles and tires and wheels, and hoses to brakes etc. . Look at it an draw it up an then test fit cardboard to see if everything works and modify until it does the job to your satisfaction. Just make it strong enough to contain an exploding tire.
All this is just sitting here trying to imagine what your RV looks like underneath. Look at it and take your time, and make patterns and remake or modify them.
We had a small machine shop near her in 2005 that I could get heavy gauge aluminum pretty cheap from or through them.
My Bother is a Design Engineer now, and a Machinist and Welder. I intended to recruit him in the bending and welding and mounting details.

down home, what are "wheel cans?" I am always interested in ways to reinforce my wheel wells against a blowout, but I looked up this term and came up empty. Thanks for clarifying -- sorry if my question is naive.