First, thanks John&Diane
for your comments and suggestions.
We actually FOUND the adapter/harness we need, and it was with all the stuff that came with the RV!!
We had remembered we had a few odds and ends that we didn't use right away or didn't know what they were for. The more we thought about it, we figured maybe this illusive adapter could be one of those things.
We found the box we'd put those things in - inside the RV - and there it was! It has a plug-in connector on the one end that plugs into the connector that is tucked under the dash area. It has loose wires on the other end, but it is what we need.
Hubby is a retired electrician, so a bunch of loose, different colored wires sticking out doesn't even phase him. As of this morning, he has the wiriing about 90% finished and we are hopefully almost ready to use the aux brake system with our little toad!
Thanks again for the responses!
Eileen in AZ