3 sets of batteries in 7 years, to me, indicates: cheap batteries; misuse; bad maintenance. Or a systemic electrical problem. Most pairs of 6 volt batteries I have had have lasted about 4-5 years each. Might want to take a look at your converter--is it a multi-stage converter (bulk/absorb/float cycles?) or just a battery charger?
Hang around and let us know what you find out, and good luck. Not having strong electric is a hassle, always popping up at worst times...
2016 Tiffin 40 QBH
2015 38RSSA, traded
2005 TK3 #1869, 10 yrs of memories,
2017 F450 KR--one more Ford is it
2009 F450 4x4-died; 2010 F450-retired