Many boats made in the 1970’s came with foam-backed vinyl material covering the interior walls and roof. The material provides some insulation and covers up all the nuts and other hardware. After a few decades, the material starts to degrade. In particular, the foam which is usually glued (either directly to the wall, or onto thin plywood sheels) starts to chemically decompose and turns into nasty powder. This causes the vinyl to sag, and usually prompts the owner to try replacing it.
Removing the vinyl is pretty straightforward. Use a stanley knife with a sharp blade to cut around the edges and peel it off – but be careful of the horrible powdery foam residue.
For plywood panels, you can buy replacement foam-backed vinyl. Websites which deal with van conversions sell it. Buy a big tub of PVA glue, spread it on one side of the plywood and smooth the vinyl onto the wood. You’ll want to make the vinyl oversize by a few inches. Remove the foam from the excess vinyl, cut into tabs and either staple it to the back of the plywood or use contact glue.
For the fibreglass boat walls, you’ll be left with gunky glue/powder everywhere. This is hard to remove by scraping or washing. Using a surface preparation wheel works well.
You could replace the old vinyl with new vinyl, or you could use the kind of carpet lining popular with the van conversion crowd – it’s slightly stretchy and attaches easily using spray glue.
Alternatively, you could paint the inside walls. You’d lose the insulation, which might risk getting condensation at night – the walls cool down faster than the air and water forms on the colder surfaces.