Generally PROPANE heats faster however your millage may vary. The reason is there is a limit to how much power they can feed the beast on Electric.
That said.. Campsites come in two flavors
Electricity included or flat rate billed,, Electricity metered and billed.
In addition some campgrounds "mark up" the electric (illegal in some states).
So in assorted places on the web, including this forum, (Though I do not have a link handy) they have charts that show assorted prices for Gas (propane) and electric (KWH) and if you run your finger down and across and are Above a linem, One is cheaper, below that same line the other is.
Oh and in some cases.. if you need hot water very fast or more than the heater holds, IF your heater allows, You can use both Gas and Electric at the same time for Fastest recovery.