Solar would be an awesome way to go, the cheaper way to go would be a generator.
I have generators, it's just nice to be able to just flip the light switch if you need something out of the trailer after dark. I actually just solved the problem by splicing into my emergency break away battery. What I found is that my charge circuit in my 7 way connector wasn't even connected to that battery so it was never getting a charge. I had been towing the trailer the last few years with a bad battery. The only thing the charge circuit was running was my interior lights which consists of 2 dome lights and a switch. So, I just spliced that circuit into the brake battery and I picked up a new battery today and installed it. I realize that it's probably not the "best" solution and the brake battery should be on its own circuit but it's got to be better than driving around for years with a dead battery. :) I figure I gain 3 advantages to doing this:
1) My emergency battery will get a charge while the truck is running.
2) I'll be able to tell if it at least has some charge by turning the interior light on to see if it works and
3) I now have interior lights if I need them without the truck having to be running (runs off the emergency brake battery).
It's possible I'll think of a very good reason for not doing this at some point but right now I can't think of one. :)