The reason your fridge goes out when driving is that it's not designed to be
operated like that.  That's what the 12V feature is for - otherwise it
wouldn't be needed.  

It's generally considered bad practice to drive with your propane regulator
turned on.  If you get in a wreck or even if you are driving on a bad road
with gravel, etc.  it would be easy to damage the propane tubing and end up
with a fire.  

The 12 V feature does draw quite a bit of current, but if your altenator and
battery are in good shape you should be OK unless you leave the fridge on
when the engine is off.  If you are going to use the 12V a lot, you should
get the 2 battery set up with a battery isolator and run the fridge off the
non-ignition battery.  

Let me throw in a couple of  more thoughts for you - this isn't the kitchen
fridge in your house - and no way will it work as well - even on 110V.  So
you have to make do the best you can.

Make sure you cool the fridge off well using 110V before you leave on your
trip..

Don't try to make ice with the fridge - won't work.  Use block ice in cooler

Freeze some of your 2nd and 3rd day consumables (hamburger, etc) and carry
them in the fridge.  As they thaw they'll help keep the fridge cold.

Good luck!

leonard jones