I haven't been able to find a 1990 manual, but there MAY be one in the library that I missed. I found one for 95-96 models and the wiring might be the same in your trailer. If you look on page 19, drawing A, in the manual here:
https://www.hilotrailerforum.com/f49...-towlight-621/, you'll see that the lift system looks like it MAY get its power through a 15A fuse, which only protects the solenoid circuit, which is a fairly low Amperage path. It's a black wire connected to fuse #2. ALL of the there fuses seem to be connected to a buss bar that looks like it receives its power through the Circuit Breaker (CB) which is probably in the battery box. The lights seem to be wired to fuse #4. Fuse #1 is the water pump and Fuse #3 is the bath fan. All of the fuses in this diagram show only ONE wire out to an appliance. Do you have any that have more than one wire connected?
You also don't have a "Master Switch" in your trailer if it's wired like this one. Do you have the Safety Bar indicator light?
Sorry I can't be more help than this. The wiring in these earlier trailers is completely different than the wiring in mine.
- Jack