It is a propane powered device and the electric is an alternative. How much would it cost to run on propane for a month?
I can go two weeks easily on one 20lb tank running fridge and cooking every day twice per day and I bet the cooking uses more propane than the fridge, so guessing $20/month on propane to run fridge..
Humidity has no effect (less than 2%) on cooling, see other post.
Are you keeping it out of the sun? Good airflow over the coils?
Consider a Moffset {sp?} (actually Danfoss, mofsett is for audio…)powered DC electric compressor fridge if you want to run always on electricity. These can run on solar and one battery with no need for grid.
ARB, Dometic and others make these.
I would think that if you plug into the wall it would cost just cents per month to run