I have an eberspacher that I fitted to the T4. Its almost too powerful for such a small cabin but it’s ace. It does take some battery juice though so Im considering giving it it’s own dedicated cranking battery – it’s not Ah that matters but it’s getting the amperage to the glow pin for 30 seconds – and fairly long, not very thick cables to my leisure battery doesnt always manage it and I have to run the engine for a minute and switch over to van battery just to start up the heater – once fired it takes very little elec.
However, as you say, theyre quite pricey.
An oil filled heater might will be fine but a bit bulky. I have a 400W “halogen” heater which could run for a while through the inverter and worked well. But it’s cheap, not thermostatically controlled, dangerous (prone to falling over – weighs nothing) and still quite bulky.