What do the spark plugs look like? Sounds like you have a leak especially if the motor isn't smoking a lot. Other than using a lot of oil how is the motor running?
Looks like the trailer was galvanized then welded on. I've been using this stuff on my galvanized trailer. I'm not affiliated with master coat I just found that this stuff works for me.
https://nomorerust.com/store/rust-stop-paints/8-mastercoat-pemanent-rust-sealer.html
This is what I was going to say. After I bought my E bike I researched locks and was going to buy the best lock then I learned that there is none. They can all be cut in seconds with a cordless angle grinder.
When I was shopping for an outboard I saw several "cheap" new outboards from several overseas "dealers". I contacted Yamaha about the "cheap" outboards and I was told that there are no authorized dealers and not able to buy or sale Yamaha outboards. The guy also said IF they actually sold...
Are you 100% positive the power switch on console is wired correctly and you didn't connect the hot 12v lead to the switched side? Are all accessories that are wired to the new fuse box on all of the time when you connect the battery cable?
Towing 400 miles is a big deal, you didn't mention if the route is all flat freeway or bumpy mountain roads or what you're towing with. I wouldn't do it without the proper trailer and tow vehicle but that's me.
