Propex sealed combustion heaters are very quiet and efficient. And they include a matched electronic thermostatic controller to avoid big temperature swings.
Be aware that Propex heaters, like the Truma combi, are rated by output BTUs, whereas traditional US RV furnaces were rated by input gas value consumption. When adjusted for combined cycle thermal efficiency the larger Propex HS2800 has about the same effective output as a Suburban 16K BTU forced air furnace. However the Propex is quieter, more efficient, and has slightly less than half the electrical load on your battery.
Because they are so small the Propex units are often added to VW campers, and are available from Westfalia upfitters such as:
Westy Ventures / Propex heaters
VW Vanagon Propex Heater Units and Systems