Does the relay look like this:

If so, here's the wiring

Power Source is #30. You need a testing power supply lead, ideally with a little breaker (say 10-amp) or fuse in it, to power #30. Once you do that, IF your relay has #87A (some don't), your test light will illuminate.
If you apply +12VDC and Ground to #85 and #86 (doesn't matter which) then your test light on #87A will go out, but it will illuminate on #87.
The relays are available everywhere! Radio Shack if you can find a store anymore. Auto Supply if you can get somebody who knows what they're doing without trying to find Year, Make, Model and Engine in a computer without knowing what's on the shelf or what it does. Amazon has bunches of them, often with new sockets.
The BOSCH brand for these was sold to TYCO, so those two actual suppliers sold/sell the best "Bosch" relays. But when you can get 5 of 'em for $15 with sockets delivered to your door (Amazon), you can have spares. It's an RV after all, not a Moon Shot.