Battery questions

If your battery won't hold a charge it needs to be replaced.
Your vehicle should charge your battery when the engine is running if it's wired properly. This normally doesn't provide much of a charge but it helps.
You MUST have a battery on the trailer if you are towing it on the road. If the trailer ever comes unhooked from the tow vehicle the on board battery will operate the brakes on the trailer.
Since you will be buying one you need a deep cycle battery, not a car battery; they can look similar.
If you always camp where you can plug in get a small cheap battery like a group 24. If you like to camp where you can't plug in and so must rely on your battery for power you probably should install a larger battery bank consisting of multiple batteries. You usage patterns and budget will determine the proper size.

Search on the Internet for '12 volt side of life' - it will be a good introduction to battery care and maintenance. Then take your battery to an auto parts store for testing, but chances are that it's shot now. Once a battery is deeply discharged it suffers damage, and if it is really, deeply discharged it is likely useless. If you have to replace it, as others have stated, ask about a true deep cycle (not a marine battery, which is a dual purpose)

- Randy

Read The 12 Volt Side of Life.
It will help you understand batteries.

Welcome to the forum.
There is a lot of information on different batteries and combinations if you do a search. You do need a battery on your TT to operate the trailer brakes in the event of a disconnect.

Get yourself a good deep cycle battery or two. Problem over. I have two Interstates at the moment. Good luck.