when the fridge is turned "off" or switched over to ac or dc, the gas supply should turn off at the back of the fridge...a solenoid will close a valve.
the dometic fridge has a manual shut off valve, you use a screw driver.
I'll remind you as nicely as I can that we all have different models here- so maybe you have a dometic, maybe not- I dunno
the name of the manufacturer and model number should be a sticker inside the door- you could look at that and share it in your post(s)
if you have a dometic, you can download a troubleshooting manual- go here:
www.nancyemmert.com/region6/
click on resources
lots of good stuff there- the 4th link down is the dometic manual
page 20 of 34 has an illustration of the dometic gas burner and shows the manual shut off- with that you can be certain that the gas supply to the fridge is off.
gas is dangerous, make SURE you have working gas, smoke & CO detectors in your van
Mike