Despite a previous poster stating "only" a bad alternator will "kill" batteries, temperature is a factor as mentioned, vibration, and power draw are also factors. You need to check for parasitic battery drain. It is has been found that accessories can stay on drawing power when the vehicle has been shut off, causing a low/dead battery condition at the next startup. Beside having a matched set of the correct size battery is always recommended.
An easy test with no skills required would be to disconnect both batteries for several days and see if truck starts when connected to eliminate or prove a shorted cell in one of the batteries. One bad cell in both batteries could draw down the other battery.