I've been chasing leaks for 5 months now in an empty shell... Its hard enough to find them that way- cant imagine trying to chase these with the interior skins & furnishings in it...
I've found my worst offenders have been the vertical seam between the endcap and fuselage. Particularly on the curb side. The water runs off the awning rail like a gutter, and develops a river down the seams. Drove me nuts trying to figure out where it was coming from, so I finally drilled out all the rivets, jammed it full of vulkem, and buttoned it back up. FINALLY leak free there... Free's me up to devote more detective work to find the others.
Sealant across the tops of the windows and doors are easy to see and fix. Rubrails and awning rails are also easy. BUT.... Pesky rivets here and there would be impossible to find without the interior out, unless you pressure test.
So... since I'm only starting from a temporarily leak free coach, my long term solution is:
#1- try to seal up the c channel in the floor and provide a way to direct the water out. There is someone on here that put some tubes in the c-channel. I think that's a great way to let the water out that will inevitably get in through a seam or rivet.
#2- Keep it covered when stored.