It sounds as though this did not have a 12V system, or much of one, to start with; the loose wires for tail lights etc. would be 12V wiring, and powered from the tow vehicle and completely separate from the 120V system. Assuming you have electric brakes on the trailer, it would be wise to install at least a small 12V battery and switch etc. for a breakaway braking system (that would apply them should the trailer somehow become detached from the tow vehicle). Many states require such a system, at least for larger/heavier trailers.
It probably is obvious, but if the water pump is a 12V pump, don't connect it directly to a 120V circuit if you want it to continue to work. Virtually all RV water pumps available today are 12V DC devices.
I would suggest at least thinking about putting in something of a 12V system with a couple of interior lights etc. and replacing the electrical distribution panel, assuming it's just two AC fuses or circuit breakers and nothing more, with a small RV panel that has AC breakers, DC fuses, and a converter (to power the 12V system and charge the 12V battery when hooked up to 120V shore power).
This PD4045 is one example of a good unit to consider. It would give you space to have more AC circuits (which, while I don't think absolutely necessary, would be better.)
Do make sure you keep the ground and neutral busses separate in the RV. They are not bonded together at the RV panel, unlike a house main panel. The entire RV acts as a sort of sub-panel to the campground or whatever it's plugged into.
