Do you need documentation, or just a number? The former is likely next to impossible... you might look on the title- some states, like AZ, include it there. But if all they need is a number, that's easy- give them one. My parents paid $2K for a largish tent trailer, brand new, in 1971, so I'd guess the Compact Jr., being hard-sided, might have been a bit more, say $2250.
The more important issue to discuss with your agent is how they will calculate its current value in the event of a partial or total loss. Not all RV policies are created equal, and some aren't worth the paper they're printed on, especially for vintage fiberglass trailers.
There are quite a few threads on the subject of insurance in the "Money Matters" section. You could check a few out before talking to your agent.