For portable use, such as with our minivan camper conversion, there are many water heaters that would work easily, but the one we recommend is the Camp Chef HWDS Triton Water Heater. It is a portable unit that takes up very little space for storage. To use it, all you have to do is hang it near the shower enclosure, and connect a hose to it from a pressurized water supply. It uses batteries for direct spark ignition to ignite the propane supplied from a small one-pound container that attaches to it. You have a 6-foot hose with a shower wand with an on/off control on it. This is an “on demand” type water heater, which means that when you aren’t using water, the unit shuts off to save both gas AND water. As soon as you open the valve on the shower wand, it instantly start up to heat the water as you use it.
The pressure switch is set relatively low, so even a gravity fed water supply might work, although it’s not recommended, in case the unit runs dry. But it will work with a normal RV demand pump, too, so if you have that type of pump in your minivan, you could easily run a hose off your sink faucet to the water heater and use it that way. As long as your sink faucet has an aerator on it, you can get an adapter at any hardware store to connect a garden hose to it. For using it for the water heater and shower, you should use the white hose for potable water.
The nice thing about this water heater is that you can use it for other than showers, too. If you need to wash dishes, it can supply hot water to your sink, and you have a handy spray wand for rinsing. That’s why you want to use the white water hose with it.
For other purposes, such as backpacking, a portable heater that does not rely on incoming pressure would be better. Zodi is probably one of the best suppliers of such equipment. If you do a search for that brand name with “portable water heater” I’m sure you will find all kinds of them. And if you see one that you think works better than what we recommend, that’s fine. If you’re out boon-docking in remote areas, I’m sure that one of theirs would work just as well, if not better.