It all depends - if your electricity isn't metered, the heat is effectively free since you've already paid for the electricity in your site fee. If it is metered, then (ignoring the convenience of not having to fill propane tanks) the economy comes in being able to select the least expensive heating medium in one's particular circumstances - either electricity or propane.
We've been down to freezing, and the CheapHeat system along with the fireplace keep the MS at a comfortable 71 degrees without any problem. The furnace is still cycling, so it has heating capacity in reserve. The only propane we use is for the range, and we don't use much of that as my wife does most of her cooking in the convection microwave, and the residential fridge is powered off the batteries by an inverter when we're traveling.
Rusty