Batteries dying overnight

I've had this happen a couple times in the last two days. The other day I was blowing out the vents with an air gun and blew out the blower fan and around the dash as well. Not sure if that matters to the story but that's when this problem started.
The next day, I went out and low battery alarm is going off. Only thing that should have been using battery power overnight is my GPS and sat radio. I thought maybe I bumped a switch when cleaning but everything else was off.
So I boosted and it started up. Ran that day...my voltmeter was showing 14 on the gauge and showing it move between 13.5-14.5 on the DataStar.
Next morning it fired up...but seemed to turn over a little slower than usual.
Last night, I shut off the battery disconnect switch. Figured if something was draining batteries overnight then that might stop it.
This morning, turned the switch on and batteries were dead again and needed a boost to start.
I guess my next step would be to disconnect the batteries and clean the terminals. I have a load tester to check each battery separately to see if I have one or more bad ones.
Cables on the starter are tight and clean (switched starter about 6 months ago) and wires on alternator are tight.
Truck is out working right now but is there anything else I should look at while I'm messing around?

 

I should be able to tell that with a load tester right?

 

My batteries are a couple years old and when I was long hauling....they were used for inverter and bunk heat. Also,now I'm working local and the roads are terrible. So I guess they could be getting weak...or damaged from vibration

 

Good idea. I think I'll load test my batteries and if they pass...I'll do the drain test. I might do it anyways. Then I'll know for sure if there's any other issues. Thanks