1. There are both combo deep/cycle starting batteries AND pure deep cycle batteries available in 12volt. You do NOT have to go to 6V to get true deep cycle as previously stated.
2. I do NOT recommend going to a dual 6V battery plan IF you are only going to use 2 batteries for the simple reason of redundancy. If you add 2 12V group 27 or 31 batteries you have similar amp hours to the dual 6V battery set up.
But if ONE 6V battery fails...you are without a house battery system. If one 12V battery fails...you still have one to use. I AM a fan of 6V batteries when used in groups of 4 as they provide a lot of amp hours for the $$.
3. There is NO reason to go to the extra expense (more than 2x) of AGM concorde lifelines vs. standard wet deep cycle batteries unless your use pattern and convenience desires outweighs the price differential.
4. No reason to throw away your existing batteries until they are near the end of life. Suggest you fully charge them overnight. Then disconnect them from the coach (remove negative wire) and wait 12-24 hours
. Don't shortcut this step!! Then put a volt meter on them. If the voltmeter reads 12.5-12.7 you are in good shape...spend your $$ elsewhere for now. 12.3-12.4 ....getting weak...showing age but still serviceable. 12.2 or less...time for new ones.
Alternatively, if you have wet cell batteries now, you can get a turkey baster and take direct readings from each cell several hours after last charge.
Hydrometer reading of <1.210 specific gravity means replace. Test each cell.