I've got an Electrolux RM212, so this is all based off that, but should be similar...
If you're planning to run it off 12v, it's worth making sure it's actually been wired up - mine wasn't when I bought the van, I guess because the older fridges use so much power that a lot of people don't bother. And if it's wired, make sure that the wiring and any inline fuses between the fridge and your battery are ok (and up to the job, my fridge draws just under 10 amps on 12v iirc). Then you should just need to switch the fridge to 12v/battery mode on the 3-way switch on the front of the fridge. There's no thermostat on 12v, it's always on, so be careful not to flatten whichever battery you've got it wired too (especially if it's the one you're planning to use to start the van the next morning!).
Being level whilst pitched is important - the coolant needs to return to the electrical/gas heating point, otherwise the fridge won't cool. Mine's bloomin temperamental, now got 4 chocks and a small spirit level to make sure it works!

But whilst driving, the sloshing motion of the van should normally be ok to keep the coolant moving over the heat source. I prefer not to run on gas whilst driving, hence why I wired up the 12v.
HTH!
Ben.