I am sure someone with a great deal of experience re-sealing RV roofs will be along shortly but I thought I would throw my two cents in anyway. In my line of work (aircraft mechanic) we never
seal over top of old sealant, it is just a bad practice and will leak again. Thankfully the sealant used on the roofs of RV's is nowhere near as hard to remove as it is from an aircraft fuel cell.
I say start fresh, clean the area well, remove all existing sealant except
in an area where it would require disassembly, such as faying surfaces or sandwiched/laminated assemblies or joints. Clean the area well with whatever cleaner is approved for your surface(s) and if available, I would use some sort of adhesion promoter.
I know I have read a lot about the sealing tape that is popular (eternabond I think?) and they talk about putting it over top of old sealant.
This is only what I would/will do, your results may vary.....