Crack seal kettle fuel
Not sure what kind of kettle your using.
We run a 180 gallon Marathon Road Maintenance Equipment kettle, and even on the coldest day, we will still only use about 120 lbs of propane ( one large 100 lb. tank, and a bit of the 2nd tank)
The burners for natural gas and propane are different as well, making it impossible to run both fuels in the same burner unit.
It is also illegal to drive with a kettle burner on.
Sometimes there are no short cuts.
Price your work to allow for all of your costs, .......and stick to your pricing.
And most of all, work safe at all times.