How do I set switches to run on 12 vdc while on the road?

I would start by checking your 12 volt battery voltage, should be 12.5 to 12.8 voltage reading, as with out it the heater will run briefly and then shut down. There are many factory trouble shooting guides on line for free. Not sure if you have a ATWOOD Suburban heater?
Normally if the fan motor comes on... it blows against a flat plate called a flail switch. That switch tells the ignition device to give spark to the GAS CHAMBER. So my guess is....with your start stop condition....the the electronic control is not working correctly due to low battery voltage. As you said....when on 120 volts going to the 12volt invertor...things work correctly. Lots of information on U-Tube on furnace troubleshooting! Good Luck Goodn

If you have a 12volt standard electrical system in your vehicle.
It is both positive and negative. The travel inverter converts this 12 volt DC current into 110v 60hz AC current so you can plug in your electronics that require standard household current to charge/operate.
There is a built in USB port if you device will accept this for 12 volt power. It also has the power oultet with ground for 110v 6hz power.
Link below has a good picture and description. Hope this helps!

http://www.sears.com/diehard-120-watt-ultra-slim-travel-inverter-with/p-02871520000P

The dsi light is powered by 12 volts, either from trailer converter, or a plug in to a receptacle that converts it to 12 VDC. It sounds like a faulty switch, if you switch it to off and it doesn't go off. . Temporarily, you can pull switch out and disconnect 12 VDC power at spade connector on switch. Be sure to wrap electrical tape around bare spade (female) that has incoming 12 VDC, until you replace switch. Is the water heater actually trying to ignite on gas when light is on? If you don't want to pull switch, look for fuse in fuse panel for water heater, or under counter or wherever, near water heater area for an adapter plugged in with wires running to the water heater and simply unplug the adapter at the receptacle.

Ah, the problem of the ages. There are 2 solutions. One is a transformer of the proper size, and the second is to purchase a heavy duty battery charger (that runs on 220 volts). Connect the red and black (+ and -) wires to a 12v to 110v voltage inverter, similar to what you would plug into a 12v auto 'cigarette lighter' socket to produce 110v current from a car battery. The charger turns the 220 vac to 12 vdc, and the inverter turns the 12 vdc into 110 vac. This works, as it is what I did when I moved from the US to England.

The turn signal module gets it's power from the accessory fuse. If the fuse is good, check for 12 volts at the #2 position on the turn signal module (located on the top fork bracket). On the turn signal module the #1 wiring pin position is ground and the #2 wiring pin position is 12 volts dc. If there is no voltage using those two pins, check for resistance between pin #1 and engine ground. If resistance is greater than 1 ohm you have a broken or poor ground wire. If resistance is less than 1 ohm check for 12 vdc on both sides of the accessory fuse. If you have 12 vdc to only one side, replace the fuse. If you have 12 vdc to both sides of the fuse you have a broken wire between the accessory fuse and the turn signal module. If you do not have 12 vdc to the accessory fuse you have a broken wire from the ignition switch and the fuse block.

12 volt is not enough power to run a fridge.

You say that you have 12 vdc when pushing the switch to up but no voltage in the down position. Switch your voltmeter leads + and - at the motor when you push your down switch and see if you have 12 vdc then. If so then you have a bad window motor. Your up and down button switchs the polarity to the motor to go up or down.

Using a voltmeter, the resisted voltage values for the fan motor are as follows:
LOW- 3 VDC
Med LOW - 6 VDC
Med HIGH - 9 VDC
High - 12 VDC

The fan resistor gets 12 volts from each position of the fan switch, and the 12 VDC is then lowered thru each of the 3 internal resistors inside the fan resistor. My guess is that the resistor has failed, and the resistance wires contained on it may have meshed together. The resistor is on the black box between the blower motor housing and the center of the dash. 2 phillips screws hold it in. Easy to remove/replace. Hope this helps, and thanks for choosing FixYa.