Actually was not problem stay 6 night and had showers every day, thanks to the one 50 gal gray tank for bathroom and second 50 gal one for kitchen. Yes you do have two campground there with hook up, Idaho Dunes RV and Sand Hill resort. From Sand Hill you really need to go long way to reach big dunes, but is nicer. But if you want to go next year for UTV Invasion event I am pretty sure you should try to book the full hook up space now...if is not late already. In BLM park where I stayed you do have just electricity hook up, but they have a dumping facility in day use parking lot area , if needed , but you can't make reservation there .
milos