Aeronautical wiring. Sorta familiar with that. All the way to cryogenic level. Any alloy solder is absolutely forbidden for cryogenic exposure. Mating materials MUST have thermal co-coefficient's that are suitable from surface ambient temps, to as low as -459.7F all the way to re-entry temps in excess of 350F.
But absolutely copper-free welding wire demanded by nuclear gamma ray resistance is sorta non justifiable for a trailer hitch. This is just as relevant as declaring soldering undesirable because of cryogenic sensitivity.
FAA/PMA is stuck in the 1940's. I got lectured many decades ago about my "audacity" to flaunt my responsibility regarding the soldering of a voltage regulator plug to a dual engine aircraft with Lycoming engines. I put my PMA number on the repair performed in Mexico. The smartass investigator was rocked back on his heels when I asked him "Have you ANY idea at all about the number of SOLDERED CONNECTIONS inside the alternators and Prestolite starters?"
A properly soldered termination, sealed with dual-wall heat shrink tube AND supported correctly with a ADEL-type strain relief clamp is damned near invincible. Properly crimped and sealed terminals are excellent but the difference lies in the environment. Tropical salt air is nasty. Leave a ready to assemble conductor and termination unsealed for ONE HOUR and you is scaroowed sonny. That's plenty of time for salt to accumulate and then starting to have it's way with the wire just when you slop on the last coat of preservative. Six months / a year / the connection will eventually fail with any kind of load on it.
Laurel & Hardy grade construction really rears it's ugly head down here. A few thousand miles on Mexican roads then park near the ocean will bring out the ugly in RV electrical construction.
