Re: Sulfur/ rotten egg smell
Disconnecting the house battery should be no problem. The easiest way is probably at the separator if your van is 2005 or newer, or the isloater if earlier. Make sure you have some electrical tape as the wires will still be hot from the house battery and you don't want arcing on the way to the shop.
If you have a separator you should just be able to disconnect the negative wire from separator to ground, this will keep the separator from closing which will keep the house battery from charging off the alternator.
Otherwise negative is your best bet, which should be somewhere on the frame, coming from the house battery or through the floor.
Last I wouldn't pull any positive cables, probably a bad time to create a spark. Also try to remove any load from the house battery before disconnecting if you can't go the separator route, as load causes sparks.
HTH, follow at your own risk (I'd open all the windows for 10 minutes before proceeding...)