Sorry I didn't get more in depth. Here is a little more depth.
First thing is identify what is leaking. It could be many things but typically it is going to falling one of two categories.
1.Seams: this includes all seams and around any penetration in the roof surface.
2.Holes or mechanical damage.
The fixes for the first one are the same and depending on how much you want to spend you could either buy a good quality eurethane caulk(please NEVER us silicone) or you could choose to "strip in" the seams with either a good quality coating and some fabric tape. The use of a good quality butyl tape is also acceptable. If you choose to use coatings you must prep the sruface first.
As for holes and mechanical damage. Any holes large than 1/2" in diameter should be patched with the same material the trailer is built with ie tin use tin, aluminum use aluminum. The mixing of aluminum and galavinized metal can cause electrolisis with causes premature oxidation of the galvanized metal. Any small holes (less than 1/2")can be patched with caulking(once again I stress the use of a good quality eurethane chaulk) ATTENTION: Before caulking any surface make sure the surface is dry and clean of oil and or debris.
Now if you are thinking about completely resurfacing the roof with a good quality coating here are a few tips. First clean the roof, A solution of muratic acid and water can be used to etch the metal to recieve the coating, regardless you must powerwash the roof using a powerwasher that reaches at least 3000 psi. You need to remove all surface grim and any oxidation, that is why a recommend a low concentrated acid solution.
If you wanna turn your camper into a complete piece of crap here are some tips:
1.Smear black roofing cement all over the top, heck noone will ever see it.
2.Smear silicone caulking all over the top, that way nothing else will ever stick to it, including most quality roof coatings.
3.Pour fibred aluminum roof coating all over it that ways it won't rust(ya right)
4.put a blue FEMA tarp over it, this is a sure fire way to turn that camper into a POS. Make sure that you use bailing twine to tie it off.