Nobody answered your question. lol.
You can go a month or more if all you do is use propane for the fridge. You might go a month using propane for fridge and the stove. It is the water heater and the furnace (not needed in summer) that gobble gas.
Don't keep the water heater on all the time, just turn it on, heat water, use, turn it off.
There are devices that pair the AC units so that only one will run the compressor at a time. so, both AC units are on, always blowing fan air, but only one is actually running the AC compressor at a time. This is all I know as I have never had two.
You might be out of time to try to investigate and install this for your upcoming trip.
Propane for a fridge is literally about the size of a candle flame. You will not use much propane at all. It is not even worth posting a forum question because it uses so little propane.