We stayed here in October when the water was turned off and the sites were half price.  There were no services at all including water, electric or dump station.  Regardless the sites are all amazing.  The park is very fun and interesting.  Depending on how much hiking your want to do will determine how long you stay.  We did limited hiking, arrived early in the day and only stayed one night,   We took the full auto tour and saw so much.  We had no cell service at all, even at the visitor's center and no WiFi.  Campfires are strictly prohibited.  If you tent camp, you will be laying on a lot of crushed lava rock so bring padding.  It was FCFS in October but plenty of spaces were available.  I would definitely come again in shoulder season, I think it would be very hot in summer.