WHAT TO USE TO SEAL YOUR UNIT.
The proper sealer is necessary for a good seal that last. Almost all doors, windows, trim, hatches and accessory covers on RV's are sealed with a polyurethane sealer that really sticks, firms up well and remains flexible.
Roofs are sealed around vents, hatches, AC units and edge seams with a product called Lap Seal. This is thicker, very sticky and usually is used to overlap the spot being sealed by 1.5 Inches. It comes in Self Leveling and Non Sag formulas in a caulking tube.
The best Polyurethane sealer I've found is LOCTITE TUB & TILE SEAL & BOND, aka POLYSEAMSEAL. It's easy to lay a good looking bead and to smooth with a wet finger where necessary.
I found it at home centers in both White and Almond. This also works good on the interior around sinks, showers and counters.
For the Lap Seal roof product, I found it at MOBILE HOME DEPOT [www.mobilehomedepot.net
]. I purchased Non Sag formula and it works great. No leaks with this stuff.