YOu said you have 3 75 AH (Group 24?) batteries. I assume those are MARINE/deep cycle
You have about 30-40 usable amp hours of power before those batteries suffer damage. (I'm ignoring the 4th one but you can do the math)
Now if you replace them (when the time comes) with TWO GC-2.. Six volt in series (and a wood block for or plastic box for the 3rd or you can drop in another Group 24 if you like)
You get (For the two) 220 Amp Hours more or less and can safely use 1/2 of it.
I have Deka G-20's (230 amp hours) in my RV. In parallel with some Group 29s
I use the Group 29's due to where they are (Low gassing maintenance free batts) but If they are where the factory wants them the GC-2 is hard to beat when it conmes to BANG for the BUCK
