Well there are some gas heaters, usually ment to consume water right away (for showering), and not pump it around. But it may work.
I don't like the gas solutions though. Sometimes people get poisend by them in their camper van, with really old and s***ty gas heaters.
Having said that, most homes here are gas heated and we don't have too many problems with it. So of course it can work fine.
(however, it doesn't works so good anymore when you cross like 15000 ft altitude or more

)
The Isotemp / Isotherm boilers that I mean use a heat exchanger on cooling liquid. They are in fact typically used in boats, or really luxiourous campers (RV's).
However, since it is cooling liquid, it doesn't matter if you have petrol or diesel.
Next to that, there are basically two brands of petrol or diesel heaters (Webasto or Eberspacher), but they just heat and pump cooling liquid around to pre heat engine. I'm going to use that in combination with the boiler.
Another thing is that you have some cheaper boilers which are basically boilers around an empty cilinder, in which hot air (from heating) should pass. Not so efficient, but it works. Especially in combination with petrol or diesel air heaters which is another story.(again Webasto and Eberspacher)
And all of these boiler usually also support an electrical heating element with lower power (like 600W or 800W) that makes it suitable to connect to low power campsite connections that many have in Europe. (usually 230Volt and 4Amps). But hey, that's not so interesting when you go bushcamping.
That is another option, just try to find the smallest electrical boiler you can find that has low power (like 800W). Maybe they're ment for in homes, but you can use them in a car as well. And then have two big auxillery batteries (so not the starter batteries) and a power inverter that goes to at least 1000W, and do everything electrically. But it will take quite some power.
Please see this page about the Isotherm water heater using cooling liquid. The smallest one (Slim 15) is 15 liter and has a square shape which makes it easier to fit.
http://www.webasto.com/int/markets-... that's enough about water heating I suppose
