How do the walls look? Any "pimples" (mold) in the wallpaper? If so, you have a roof or window leak. Do you have a slide out? If so, check the edges underneath the slide for soft areas. If so, that may be the culprit. Also check your wheel wells. Could be water intrusion while towing in rain.
I wish you a simple solution. Good luck!
2015 Jayco Jayflight 27RLS
2012 Ford F-150
2001 Fleetwood Terry 28x
1996 Viking Pop-up
1979 Coleman 8 man cabin tent
1967 G.I Joe (hasbro) canvas pup tent