Here are a couple suggestions about the walls and exterior. I use Mr. Clean Magic Erasers to clean the plastic surfaces at least once each season. Use household detergent on the easy stuff and Goo-Gone or something strong on tree sap. Careful working on the roof... Hoses, water and ladders don't mix well.
Once clean, I started using a high quality UV protecterant laveled "303", "Marine/Recreational Clear Vinyl Protective Cleaner", found at boat sales and repair centers. About $13/bottle but is reported to be the top product for protecting kayaks, boats and other plastic surfaces from UltraViolet degradation.
AS for cracks in the walls, DON"T start by trying to remove the vinyl... I haven't heard of anyone having positive results for replacing the exterior surfaces. Interiors are slightly easier.
Start by cleaning everything and trying to use a silicone glue/sealer (like for around bathtubs) to seal the cracks.
If anyone has positive experiences or suggestions they would be welcome!
Happy Trails, Jeff