I honestly can't believe we went so long without this. Our camper came with the standard six gallon hot water tank with 120V AC or LP gas operation. Considering that modern "economical" shower heads of today's homes have limited the flow of water from 7 gallons per minute to around 2.5 in the past few decades, we aren't talking about very long usage here. I could typically last around 10 minutes in a shower, but my wife who likes a bit more heat than me was lucky to get 5 or 6, and that is with turning the flow off when she wasn't rinsing. On top of that, one of us (usually me) would have to wait at least half an hour for enough water to heat back up for a second usage. If we were on vacation with a couple of kids, the park shower houses would be the only option.
After a bit or research on tank expanders and the like, we decided to purchase a tankless water heater which could supply an endless stream of hot water. We went with this Girard model from PPL Motor Homes. I'm not going to try to explain how it works, but the temperature of the water is controlled by the flow. The unit kicks on when we open a hot water faucet and instantly heats the water based on how much water is flowing. We first open a faucet all the way, then slowly reduce the flow until the the water is at a desired temperature. It was a bit tricky to get used to at first, but after a bit of practice, this has been an invaluable addition to our home, and since the unit is only running when the water is flowing, it saves a lot of energy typically spent keeping a tank of water hot.