A multimeter is the best way to check your batteries, but it sounds like a dead cell/short situation. Try to keep your batteries fully charged at all times, and even with true deep cycle batteries you should not discharge them too deeply or you will reduce performance and shorten their lifespan.
We use Battery Tenders on all of our batteries that are not in daily driver vehicles and they seem to keep things well in hand (especially over Winter, when batteries can be damaged by slow, prolonged discharge and cold temperatures.) I have installed 'permanent' cables to allow quick connect/disconnect to the Tenders and that keeps me from having to pull batteries out and put them back all the time. Obviously this may not work well for someone who has to store their vehicle in a location that can't provide power to the Tender...
When you get a replacement, check to see what the manufacturer suggests as far as discharge levels. It's probably safer in the long run to stick with that rating than to guess and accidentally risk damage.
1975 Westfalia (VW)
2014 Rockwood A122