A new 5th wheel rubber membrane roof can be pretty pricey, like in the area of $10,000. You might want to check out Henrys Tropi-Cool Silicone Elastometric roof coating. I had an RV servicer apply it to my roof 2 years ago, and am entirely satisfied. Pre-treat mainly consists of pressure washing, my 38 foot Bighorn took about 10 gallons, and was done in a day. The vendor charged me $900.
Home depot also has the Tropi-Cool caulk/sealant.