1. There is no QUICK FIX for a vintage trailer. Sealants will not help you and roof goop will ruin the roof metal, not to mention it won't seal against leaks. All windows, the roof vent and doors should be removed. After cleaning, they can be re-installed with new putty if the framing is in good condition.
2. If you see water stains and damage, there is ROT behind them. You can not repair a stick built trailer from the inside. There is no way to address the skirt and sill boards as they are below floor level. 99.99% of vintage trailers need the skins lifted to inspect and do proper repairs. Most of the rot is below the front and rear windows, lower sill and skirt boards, wood edging and sometimes rafters. (usually due to failed roof metal)
3. A sturdy floor does not mean the rest of the trailer is sound. Good interior panels do not mean the trailer is sound. (especially if there are "minor" water stains at the corners)
4. PLEASE protect yourself and others. Your trailer should be carefully inspected and repairs done before towing. If the body should come loose on the road, whoever following would be in danger. Likewise, a rotten trailer can be out of balance and jerk the tow vehicle off the road. PLEASE protect yourself and others.