I know you've already changed the batteries, but here's what I found on mine. The passenger battery was found boiling the fluid after an engine shut down and I popped the hood right away to check on something else. I assumed the battery that was boiling was the bad one so I disconnected the drivers side and drove a few days to work using only the passenger side battery, no problem. Then I unhooked the passenger side and hooked up the drivers side and tried to do the same thing. It wouldn't start the truck, that battery was almost dead. So it appears to me the driver side battery was weak and the alternator was charging full bore to try and charge the weak battery, meanwhile the passenger side was getting way overcharged, because of no isolator between the two batteries. I put a new battery on the driver side and drove it a couple of months and kept checking and the passenger side never boiled again, so just to be safe I put a new battery on that side too to keep everyting as even as possible. I always recommend once a year you pull the cables and clean up everything just to be sure your getting the best connectiion. My original batteries went 6 years, but I don't live in cold country either. Fortunately for me the boiling fluid stayed in the battery, never came out onto the battery tray. It pays to pay attention to everything on a vehicle, I check constantly to see if I can find something wrong before it becomes a problem. Just my own personal maintainence habit, but it works.
02 2500HD LT D/A SB CC 4X4 BLACK, Westin stainless nerf bars, BW GN Hitch,Racor 60S post oem fuel filter, Oil Guard bypass engine oil Filter. All synthetic fluids. Kennedy boost valve, edge, Modified air intake,EGT & Boost digital gauge,TransferFlow combo fuel & tool box, Air Lift Suspesion Bags Rear & compressor with remote, Bilsteins front & rear, Frontier front replacement bumper.