Sounds like those interior lights are fed by a 110V circuit, especially if they are florescent fixtures. There has to be a breaker panel located somewhere inside the trailer (possibly in the back of one of the cabinets?) that controls that circuit or at the very least a step-up inverter to provide the 110 volts. While it doesn't happen very often, I've seen wires severed by interior paneling screws. As was mentioned above, there should be a male receptacle, possibly under a weatherproof cover, that provides the AC power to the trailer.
Here's a link to the Owners Manual webpage -
https://forestriverinc.help/#/contin...7-6660BBE4868E
And in a pinch you could visit your local Camping World! Tell them Marcus sent you!
Good Luck with the project, Ed!