Thanks to all for pointing me in the right direction on the battery issue. It was indeed the wiring and thankfully a simple inexpensive solution. I had installed this quick disconnect last year.
Taking the disconnect out of the circuit, I now have full voltage in the TT, lights working, etc. Don't know yet if the disconnect switch itself is bad or if one of the connections, which appeared pretty solid, was bad.
May try to reconnect the switch after I get the TT back from the dealer this weekend and see if the problem reappears. Is this a common problem with these types of disconnect switches?