If you're batteries are staying charged and all the other DC devices are working, there has to something interrupting the power. When things are not working properly, break out the meter and start chasing power back from the device, through the switch, and back to the distribution panel. If the stove and meter are the only two devices on a single branch circuit from the distribution panel, it could be as simple as a loose or corroded fuse holder.
BTW, if you are having a problem with establishing a ground, grab some wire and attach one end to the negative post on the battery (clamping pliers work well here). String the wire out to your work location and use that for measuring. as ground.
