I'm also in southeastern Michigan. As far as help with restoring, it depends on what you want done:
, suspension, brakes
, towing lights/towing wiring, hitch, hitch jack, coupler, chains (EDIT: also frame issues): I would take it to a utility trailer shop. I've had good luck with Ajax Trailers in Warren and Joe's Trailer Shop in Livonia for work I don't want to tackle, and also for parts.
- Fiberglass body repairs: A boat yard. (I have been doing my own repairs.)
- Interior hardware, cabinets, walls, etc.: This is really handy man work.
- Interior plumbing, propane
: Kline's RV Center in Warren is good. They'll do the work, also stock vintage trailer parts. I would avoid large RV chains such as General RV and Camping World.
I've also worked with Paul Neumeister in Sebringville, Ontario. He ordered and replaced two torsion axles for me including fabricating and welding custom brackets. He also does fiberglass repair work. He runs a body shop. Also stocks retro trailer parts.
Also if you are looking for vintage trailer hardware, check out Bontrager's in White Pigeon.