You say this is a conversion van. That would mean all kinds of groovy gadgets that could go wrong some day. Lets try this instead of using a ohm meter lets try to find one of those old test lights that you can't use on computer systems. You know the one that has the big light bulb in it that blinds you when its on. Unhook the negative battery cable. Hook the alligator clip of the test light to the negative cable, touch the tip of the test light to the negative terminal of the battery and at the same time touch the negative cable to the terminal and hold for at least 1 minute. Release the cable while still holding the test light. If this light lights up you have a draw. As long as you hold this test light like this all relays and contacts will stay closed allowing you to locate you short. Do the same thing you did before with the meter, unplugging fuses till you make the light go out. Remember look closely because you are not dealing with a stock unit. Also disconnect the alternator trigger wire also. If you need anything else please ask.