I just did this myself. After seeing how my trailers wiring was done, I realized it had already killed my 2 Group 24 marine batteries. After testing with a hydrometer it turned out I only lost 1 of the 2.....but they have to stay in pairs so...
After moving my converter closer to the batteries (bigger job than I expected), I would have gone with 2 x 12V Group 29's, however they were about 1 inch too wide to fit on my trailers tongue. The reason I wanted them is they had about 20aH more than the 2xGC2. Those were the only two options I found that came close at a reasonable cost.
Ultimately ended up with the Sam's club energizer GC2's. Just put em in last weekend - all seems well and I'm hoping for many years of good service....