What Old-Biscuit and doughere said. You have a short between the two circuits. Remove one fuse and each circuit is still protected up to 15 amps until you find the short.
Each of the circuits is protected up to a maximum of 15 amps. The load on each circuit may only be maybe 3 amps but the designer calculated the wire can handle 15 amps so he fused it at 15.
With two 15 amp fuses in parallel (outputs shorted) it would theoretically require a 30 amp load before blowing a fuse (both would blow). But, if you pull 29 amps through one side of the shorted pair, the fuses may not blow. 29 amps in that single wire may not be a problem, but what is the safe limit for the wire used in either of the circuits.
The chances of pulling too much current is low but removing one of the fuses until you resolve the issue is on the safer side.
Might think back to any mods you made to the camper to help resolve the short, such as hanging a picture or a shelf with a screw.
A couple of years ago, I found a screw running directly through the middle of the #10 wire that supplies generator power to our transfer switch. The screw held an aluminium support brace which was in the basement and was isolated from all human contact. But, it was potentially a problem.
Might need to find your short to alleviate potential problems.
