1995 G20 Mark III Batt keeps dying

My 1995 G20 Mark III batt keeps dying, we have replaced the Alternator, had the batt tested, it was still good, yet when the van sits for more than a few hours it loses charge, nothing is turned on in the van. We have checked to see that all int power is off ex TV, VCR, Radio, Rear Fan Motor, AC, ect... and can't seem to figure out what could be draining the power. A friend of mine had a similar year model and was having the same problem and eventually installed a batt cutoff switch. I would rather fix the issue, Any help would be appriciated.

There is definitely something discharging the battery if it lasts only a few hours. Here is one technique to try and find the short. You will need a test light that you can connect between the positive terminal of the battery and the positive battery cable. I jammed the pointy end of the test light into the battery post and used the ground clip to attach to the positive battery cable. With everything off (i.e. nothing drawing current from the battery) the test light should be off. If something is drawing current it will flow through the test light to the battery cable, lighting the test light. There are normally a few odd things that always are powered up (e.g. clock, computer, etc.) but they should not be drawing enough current to make the test light illuminate. Now pull each fuse out (only one at a time, not all of them) in turn and check to see if the light is still illuminated. If there is no change, put that fuse back and go to the next one. The one that makes the light go out is the circuit that has the short. Now you will have to investigate what is causing the short (pinched wire shorting to body, bad component, etc.) but at least you have narrowed it down.
Mine was bizzare. The van had a power drivers seat with a plastic trim panel over the switches. In time the panel cracked and kind of sagged a little. Somehow it got pushed down enough to push on the seat height switch and then the edge of the panel caught on the seat fabric - so the seat was forever trying to go down. There must have been some kind of protection to keep the motor from burning out, but it continued to draw current! I snapped the panel back into alignment and the light went out. I fixed it by unplugging the power to the seat so it couldn't happen again.