Unless I'm misunderstanding your question, which is certainly possible, Diesel engines have radiators with coolant just like a gas engine does - and it's that coolant routed to the heater core in the cab that provides front cabin heat.
Perhaps you're thinking since the engine is all the way in the back that the engine's cooling system is too far in way to work in the cab - but they run coolant lines from the back all the way to the front of the cab for heat.
In our gas Class A, they also run coolant lines from the front engine back through the coach to provide auxiliary coach heat for when we're driving AND also to the water heater to heat the water in the water heater while driving so that you have hot water immediately available after a drive.
2017 Winnebago Adventurer 37F
2016 Lincoln MKX Toad