Unless you want to keep it for a while, you might be better off just doing a cosmetic clean-up and selling it now. Here are a few reasons why:
1. Tow vehicle prep. You didn't mention what you would be towing with, but if it's not already set up to tow, figure at least $200-$250 for a hitch and trailer lights
2. While there really aren't many mechanics that specialize in FGRV's there are a number of RV dealers in your area that can check out you rig and give you all the bad news about what needs fixing. Figure another $100+ for the inspection. Needed repairs and updating can add substantially to this number, especially on a 40 y.o. trailer at an RV dealership.
3. As it sounds like it has been around for a while, tires
over 6 years old will need replacement, another $150.
4. The buying season for FGRV's is just getting under way and an ad in any bay area Craig's list will produce callers within a few minutes. FGRV's are HOT in your area and sell high and fast. If you wait until after Burning Man time, prices will fall
About value???? Besides your +++ location, it all depends on condition, condition and condition. I have seen "Project" Pacers sell for as little as $2000 and restored Turnkey units as high as $7500. If is in average condition, without needing obvious major repairs, $5500/offer might be a good starting point, some may try starting higher, and you can always come down if there are no bites..... Selling it to the first buyer at your asking price often brings on the dreaded "Sellers Remorse" syndrome.
BTW: Old, Rare, Classic, etc. might be terms that help it sell, but are not plus factors in pricing.
Good Luck and let us know what happens.