Are the batteries identical and the same age? If not, that could be the problem. If they are, as said, could be a wiring issue or one battery has a fault and dragging the other down.
No need for isolation.
I had two group 27 AGM batteries from Batteries Plus that were the same variety, but with two different lot numbers and two different manufacturing dates. I
knew there was going to be trouble with them eventually, but it took six years. I kept the truck on an Optima brand float charger whenever it wasn't being driven. I won't use Optima
batteries any more. (+ terminal on one battery, - terminal on the other.)
You can park that truck for a month without draining the battery's to low to start it.
Sure there is something wrong with the added wiring / gauges.
When I took delivery of my truck in 1996, it came came with a dealer-installed alarm system with remote unlock. I liked it then and still do. It's still working. But it drains just a smidgeon of electricity from the batteries whenever the truck is sitting. Ozymandias is correct, though. Without the kind of drain my truck has, good, properly matched batteries should hold their charge for a month. Newer trucks and those with newer entertainment systems may drain more electricity to hold their settings than mine does.
I have thought about installing battery isolation switches. I still have not. There are two general types. One type is a complete on-off switch. The other contains a fuse that permits the radio, alarm and so on to continue to work. But starting current will blow the fuse. Only the first type will prevent one battery from draining into the other until they are both dead.
When my six year old batteries got onto their last legs, I disconnected the negative cables from both of them. Then fully charged them using two separate chargers. I measured the voltage, and at the beginning of the test they were within a tenth of a volt of each other. Leaving them disconnected, I let them set for 48 hours and tested the voltage again. One had held good voltage. The other had dropped at least a volt lower. The battery that held good voltage got put to use on my travel trailer's tongue. The bad one, plus a junk battery got traded in on a new pair of AGM batteries. This time I got the same lot number!