What size propane cylinder/tank----gallons??
In those temps you are getting up to a week on propane use then you are doing GREAT? Unless you have a motorhome with a large propane tank
In -14*F temps we had to get a 30# (7 gallon) filled every other day
'Golden_HVAC' makes a good point about the design of RV water heater.
That combustion chamber is wide open and runs up thru tank sucking cold air in/around water inside tank
That's alot of surface area in contact with the water.
In (-)degree temps I wouldn't experiment and just pay for the propane
