I too had my '04 Ram diesel batteries last until two years ago! I went to Seattle for two weeks and left my trailer hooked to the pickup with the trailer interior lights on.
Use a high amp, constant duty solenoid, wired to be on only with the ignition on, in the trailer battery charge line. Shut the truck off and it isolates the trailer battery from the truck.
If the results don't match the theory, change the theory.
