Petrol station should be your first clue that this is not a US rental. It's going to be cold in Iceland and all of the items you asked about us propane to operate... even the refrigerator plus 12V power for the controls. Doubt you will have a generator so going to a Camp Ground will be needed every other day to charge the battery for the camper, different from the engine. With a generator you can go several days before needing a CG to dump and get water. Stop worrying and go have a great trip and the dealer will be able to answer your questions. I would not worry about toilet chemicals.
