It could be argued the other way, totally depleting the vehicle fuel tank would leave you in a worse situation with no heat from the diesel heater or able to run the engine. I’m fine with fitting either the 5ltr or 10ltr tank secured in the vehicle, I’ve carried many cans of two stroke mix and spare petrol over the years in arboriculture, it’s not dangerous at all. It’s a see though plastic so I’ll know when I need a fill. Diesel isn’t volatile like petrol, my main concern is dripping the tiniest amount will stink the van out, so I’ll be pouring carefully with a cloth, or getting a non-drip self seal nozzle like on the chainsaw combi cans. No way I’m dropping out my vans diesel tank to drill a hole in the top, and splitting off the fuel line isn’t happening either.
This is the kit. Rigid fuel line, tank, heater, connectors, inlets and outlet ducts, fuel pump, controller, filters, exhaust muffler, mountings. Very simple, it’s a mini diesel engine that draws air in from outside, and pumps exhaust gas outside (that’s the two holes in the pic, the heater is upside down).
The clever bit is a fan that blows air from inside the van over the hot engine and pumps good dry heat is completely separated from the the combustion chamber. It’s just an air cooled diesel engine where the warmth is circulated into the van. Your leisure battery will be running the fan and the glow plug to vaporise the diesel fuel. That’s another good reason to be using the separate fuel tank, the system isn’t affected by a flat vehicle battery.