What size engine? Looking at 1996 ford E350 chassis, there were a few engines available. The 7.5 diesel probably would do better with a 850 CCA.....however a gas motor should be fine with what you have. How cold do the winters get where you are going ?
If you have to idle it for 15 minutes, that has nothing to do with the battery....sounds like the engine might need a tune up.
Starter motor could be tired also.
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