Charging a Battery Bank

I've learned of another issue that I have to address before buying new batteries. When I took my single Marine/RV 12v battery off for the winter, it was dead. The camper had been plugged in to shore power (20A extension cord) for several weeks. At the time I took it off, I didn't give it a thought. Just connected it to my battery minder and fortunately it has recovered but with all of the comments in my posts, it got me to thinking about my Converter/Charger and whether it was working properly or not. It's a WFCO 8955PEC. So I got out my voltmeter and I'm getting nothing at the end of the battery cables. It's likely a problem with the converter/charger since that make/model is notorious for going bad, but when the weather breaks, I'll make sure it's not the 6 AWG cable or both. Then I'll tackle which battery options I'll go with.