The best= Eternabond tapes, butyl is OK under trim where the butyl is held in compression. Geocel Tripolymer caulks and sealants are what I use. Proflex RV is one of those caulks. Use this same stuff for your marker lights and around trim pieces.
The staples holding my siding are galvanized steel, I bet yours are too. For framing, I glued all the framing with liquid nails and then screwed/nailed it all together. You will discover the sequence and pattern of fastening and building along the way. Most trailers have the wall framing attached through the floor sheathing. The floor sheathing is usually the first to go on when mfg., I even had carpet and vinyl under the wall sill plates.
Good luck and be thorough. You only need to do this once and when sealed back up, it will be better than new. Add good insulation (extruded polystyrene) into the wall cavities if you can.
Good luck.
