That's interesting.  I get your point. 

When I go to a motel, there are amenities offered as part of a package deal, whether I take advantage of them or not.  Maybe you need to do the same with EV owners. Maybe you need to have a separate surcharge for those individuals that come with EV vehicles.... all EV's would pay the surcharge, whether they use the service or not.  You could compile a list of known EV vehicles.  IF a person registers to stay, ask what vehicle they have. Charge accordingly.