When I got my trailer there was a rotted spot on the wood floor just to the front of the entry door. When I resolved the issue, I had identified and fixed 11 separate leaks that contributed to that damage. I ended up removing the inside skin to get access to the leaks. The last and hardest leak to find was an exterior skin panel lap joint that looked just fine from the outside.
The easiest leak to find and the most time consuming to repair was the vista window above the curbside living area operating window. Of course the obvious leak points were the roof vents, television antenna, awning brackets, awning rail, window gaskets, door gaskets, and loose and missing rivets under the awning roller.
I followed sage advise here and the service manual, and did all the replacements and repairs from the top to the bottom. When I finished, the only leak I couldn't fix was a designed leak. That was the step release slot. For that I added screened drain holes under it and added aluminum bubble insulation that is suspended above the bottom of the belly pan. I coated all of the frame I had exposed with Por-15.
I can see keeping this trailer dry and in top condition will require a religious commitment. The good news is, all the parts and supplies are still available and help and advise is free and useful.