Are you just looking for enough light to keep from tripping over stuff, or enough light to do work by?
Those small touch lights that they sell for use in closets are LED lights running off of 2 to 4 AA batteries. They will run literally for a week continuously on one set of batteries. Walmart has some cheap led lights with about 10-20 LED's intended for camping that run on 4 AA batteries that provide enough light to do work that does not demand really bright light.
If you bought a few of those cheap LED touch lights and wired them in series to a 12V gel cell they would run for several weeks continuously on one charge. You can buy a small 5 watt solar panel from Harbor Freight for about $60 that would work to charge the gel cell.
For close work that requires really good lighting like final engine assembly and inspecting spark plugs etc. you would need several of the LED fixtures to have a bright work light environment. The one problem with the bright white LED lights is that they are not a true broad spectrum light source so it is difficult to judge colors with them. I use them for general emergency lighting, and a single 3 LED lantern from Walmart intended for camping will provide enough general illumination for moving about safely but not quite enough to work by.
I would suggest you pick up a couple of the LED touch lights and one of those large LED lights for camping and see if they provide enough light for your needs, and how you like the character of the light.
You can also buy small florescent tube fixtures designed to run on battery power that in my opinion give a better (true white) light than the bright LED's. I personally find the bright white LED's a little bit harsh in the character of the light they produce. I have 2 of the small florescent fixtures that run on AA batteries that I use for working on computers, they make them in 6" long and 12" long designs which work great to see what you are working on inside a computer case. They could also be wired to run off of a 12V power supply by rigging up the proper supply voltage or wiring them in series so the total voltage required for the set is 12V.
Once your eyes get dark adapted it does not take much light to do general work so the best way to judge how many fixtures you need is to actually test a couple of those cheap touch lights inside the trailer at night. In the day time out on the salt the exterior is so bright that most any light will seem dim by comparison, so you might actually need more lighting during the day time on the salt than at night because your eyes will be accommodated to the outside light levels.
The one advantage of the small touch lights is you can put them exactly where you need the light, rather than try to illuminate the whole trailer interior to suitable light levels.
Painting the walls and roof of the trailer interior white will help a lot!!