Best screws to attach WH to foam board?

I really don't think there's much stability for screws of any type to grab in any type of foam board. I can get at the backside of my WD from inside under the dinette seats. See if you can get access from inside and use either 10/24 (3/16th) stainless nuts and bolts long enoughto hold it together.

That is going to be tricky -- the screws are running into the narrow side of the foam board, if I am viewing the photo correctly. So there is not a lot of "meat" for the screws to grip. And you can't use screws that are much fatter -- they will split the foam board.

Is it possible to remove the heater completely? (A big hassle, I am sure.) And then fill the screw holes with something like epoxy, so that the screws will have something to bite into.

You will want to drill pilot holes, of course. Which will be hard because the opening of the water heater is not wide enough to accommodate both the drill motor and the drill bit. So you might need a right angle drill attachment.

I would recommend long stainless steel decking screws -- they will probably reach undamaged foam in order to grip better.

Good luck!

Foam screws. In all seriousness, in a situation like that, I'd be adding a backer strip of wood to the inside and use longer screws. The original screws were intended to catch the fiberglass exterior surface, but it sounds like that's too far gone.

x2 you need a back plate to screw to.

Yes yes the foam screw thing was my attempt at comedy. On the left side they use metal bracing, with self tapping metal screws. Is that common practice to only brace 1 side of the WH?

Is there a high temperature spray foam that would work? That stuff is pretty sticky. I don't understand what exactly you are doing but just a thought.

If you have room, PUSH the water heater further in the compartment. If no room pull the Water Heater. Then glue 2X2 wood studs up the sides and across the top on the inside to that foam board. Then once the glue dries, pull the water heater back to the correct install in the hole. Install screws to hit the new 2x2 backers and make sure you install 2 screws thru the BOTOOM if not already installed. Then seal the corner gaps with normal expanding spray foam and install the outer trim ring with its screws. The water heater normally just has the 2 screws thru the bottom into the floor, but it is best to install screws on the sides also. There is no real force that will cause the WH to attempt to slide outward as long as the 2 floor screws are installed. USUALLY, the OEM's have a aluminum or small wood studs that frame the hole cut out for the Water Heater inside the foam wall before it is laminated. I have even seen where they install aluminum angle from the inside, but the wall is usually thicker to do that. Doug

I like drywall screws for these marginal applications.

Good Job Forest River......

Cut out opening is 'suppose to be' FRAMED with wood both sides and top

WH is then anchored with wood screws...

wopachop said, "Yes yes the foam screw thing was my attempt at comedy." Well, you got me good! Well done.

And yes, I agree that a backer board has to be the way to go. If possible use hardwood (oak?) for greater strength.