I have the Victron controller/monitors in mine and love them. Highly recommended.
Good means that you monitor the charge level with something more than what the ZAMP has. Victron BMV or Trimetric TM2030. Something that uses a shunt to measure AH in and out so it can guess at the batteries real State of Charge.
Good means that you don't discharge the batteries too deeply too often. I aim at no more than 50% discharge, ever. Rando advises that this is conservative, and deeper discharges are OK.
Good means you get the spec sheet for your batteries that lists the voltages/currents that your specific batteries need, and set the controller to use those. Good batteries (Lifeline, Trojan, Rolls) will have those kinds of spec readily available to you.
Oh, and 45 and 30%... wow, those batteries are toasted!
Vic, thanks again for your detailed reply. Unfortunately, there's a new twist to the saga.
I wasn't sure that the first place I went to to test the batteries gave me reliable feedback, so I went to another auto parts place to double-check...their test showed the batteries were FINE. I had them do it over again, and the second round of tests showed they were marginal......WTF???? I left there disgusted and went down the street to an auto repair/wrecking shop. The guy there was VERY helpful. I explained what happened with the previous two visits, and he told me he would 1) put both of them on the charger to make sure they were both fully charged before he tested them, and 2) he would use his $1500 load tester, which AAA uses, to see what was going on. Came back a few hours later and......both batteries are FINE. No need to replace.
Now I don't know what to do. My guess is that my frig is causing the sporadic voltage drop, and drawing so much current that it's dooming the system. When the drops happen, the compressor on the frig keeps cycling on and off. I wonder if there's a voltage regulator on the frig, like on a car, that could cause that. I guess the other option is the solar controller, but I think that's working fine. Today I left the camper out in the sun all afternoon, with the frig running, only on battery power, and the controller read 'FUL' for most if not all afternoon, meaning the batteries were topped off, and the frig ran consistently (I had the thermostat turned all the way up to the coldest setting). Last night when we were camping, about an hour after the sun set, the frig started its on/off thing again, with voltage drops every time. The only things we were running at the time were the frig and one ceiling fan.