what works for us.
We live in a fifth wheel, not a bus, but we've had pretty good luck with electric heaters. We use the kind that have a low wattage setting (750 watts instead of 1500) so we don't overload the circuits. They're hooked up through a thermostat (available at any home improvement store - either mechanical or electronic). Three or four heaters are all we need. We were careful not to get two heaters on one circuit breaker. Just be sure not to leave things laying in front of them.