We have used one of the coleman instant hot water units for 4 or 5years now.
Usually 8-10 guys in camp. Normally only take shower once during hunt though, some years not at all. Depends on weather and action. Coleman unit has done it's job. Will shower all hunters on one charge of battery. And never used more than one of the small propane bottles per trip. In a just a few seconds, it will heat the water hotter than you can stand it.