I use shore power to do the initial cool down and leave it on that mode until we are ready to go. I do make it run on propane for an hour or so to make sure that function works! No surprises when you are firing up the truck, ready to go.
I always run the hot water tank and furnace too for a bit just to make sure they operate.
I Keep a gallon ziplock bag full of ice cubes in the house freezer.
I put it in the freezer compartment of the rv's fridge... That helps a lot. Also put 5-6 cans of soda pop or beer in the tray below the cooling fins of the fridge compartment.
This get's the fridge cool quite quick....
I do not know how long it takes. I always start the fridge up on the day before leaving.
As for how long a 40# tank will last.
Our fridges take about 1000btus/hr when cooling..
If the fridge is in the process of cooling down it will use about 24,000 btu's (24 hrs).
There is about 92,000 btu's in a gallon of Propane.
Easy math eh? Rounding you can figure the initial cool down will take about 1/3 gallon.
A 40# tank holds about 4 gallons. Depends on the fella filling...
After the fridge has cooled down the big variable will be the outside temperature.
At around 100deg I think the fridge is in cooling mode almost 100%... So 1 gallon every 3 days...
At around 70 deg maybe it runs a little less than 1/2 of the time... You do the math.
Jim
2000 2500 9.6 Bigfoot,94 F250, Vision 19.5, Bilstein shocks, air bags/pump, EU2000, PD 9260, Two Redodo 100ah Mini's, Aims 2500 Conv/Inv, 200W. solar, Morningstar Sunsaver 15A/ display panel, Delorme/laptop for travel, Wave-3 heat.