I would consider replacing the battery if it is more than about 4 years old regardless of whether it "tests" good or not due to the fact that automotive batteries rarely last more than 4 years or so.My van doesn't seem to hold a charge on the battery. It tests fine, but every couple of days the car wont start unless jumped. then it will run fine again. Very frustrated. My toyota will sit for weeks and start right up.
Having said that, I would think that you may have a "parasitic" (i.e. excessive, abnormal) current draw problem that is draining your battery. I had the same problem with our van and was, with luck, able to trace the problem down to the optional "power outlet" circuits (see your owner's manual for more info) by removing the "12V OUT IGN OR BAT" fuse (located in the "IPM FUSE AND RELAY CENTER" block under the hood). Removing this fuse reduced the parasitic current draw down to a normal level (I, unfortunately, haven't yet had time to trace down the actual problem in the circuit or circuits).
Other Caravan owners have traced their parasitic draw problems down to glove box or other interior lighting that stays on when it shouldn't, bad door lock solenoids or switches, Hazard lamp relays, etc.
Modern vehicles with their various on board computer control systems and other electrical/electronic devices will, unfortunately, drain their batteries (while sitting) much quicker than some older and or less "feature packed" vehicles. A case in point is the fact that my old 1985 GMC S15 truck can sit for months on end and still start.