Yes - you can put a parking / night heater in, fairly standard fare on campers and day vans. Have a look on the Interiors thread for a few options, underseat, underfoor, with standard vent plastics, etc
It's best to have a 2nd battery to run the heater to avoid the drain on the starter battery overnight, also worth having some lighting and 12v / USB hanging of the 2nd as well.