The value of a thing depends on how much you'll let it go for compared to how much someone is willing to pay.
We did much the same with our 1973 Amerigo
FG-16; we paid $6000, spent nearly $3000 fixing it up, and would list it at $10,000 if we were selling it now. In a year, or five, who knows?
I'd start the listing at what you paid plus what you put in. Never mind your time and work. In other words, go for the money, not the effort. Consider that not only sunk cost, but a hobby activity that didn't work out completely.
You can always lower the price. Unlikely you'll end up with a bidding war in which the price climbs.
I'd follow the above advice and get 'er clean, put a small bowl of wrapped, soft candies on the table or counter, and a small yellow flower, like, say, a potted marigold plant.
Stage it a little. Lay out a throw blanket and a couple of colorful pillows. Maybe a nice place setting. Assuming you have a table to set it on. Make a thorough list of what you're already done. Include receipts if you have them. Make a little booklet or notebook of all the paperwork. Color-theme it to the flower, or blanket, or whatever.
I know it's not finished. That makes it harder. But remember, you don't sell the steak, you sell the sizzle. Someone, somewhere, WANTS that first-year Boler. You know they do. THEY know they do. You just haven't connected yet.
NOBODY would buy anything if they didn't think there was some advantage. You did, once, and someone else will, again. And they'll have a wonderful, unique trailer.
Be patient. You've waited this long. Be ready to go down in price from your calculated maximum, money already spent. As my economics professor insisted, "sunk cost is no cost
In other words, the money is gone. But you don't have to put up new money, either. So it's all to the good from that point.
BEST good luck to you!