You may want to start by reading about batteries at this Web site:
The 12volt Side of Life (Part 1)
The nub of your question is "which is longer lasting?" You find that almost everyone recommends deep-cycle lead-acid batteries for "house" use, such as your trailer.
Since you already have 12V batteries, you can stick with that voltage. If you decide to go to 6V batteries you will have to cable them differently. It's worth it to some people to replace 12V with 6V, and not to others.
The newest technology uses lithium-based batteries, similar to those used in portable computers. Lithium-based batteries are generally lighter, and have other advantages over lead-acid batteries- but are generally more money than most want to pay.
If you search for posts on "batteries" here on iRV2 you'll find a lot of information, opinions, and some old wives tales. Read as many posts as you can.