I've had quite a bit of experience keeping a D3 (same age as yours) running. They are actually amazing simple/efficient heaters.
Two most common (for me) issues were - the heater was above the fuel
- if the hose clamp was slightly loose, it would allow fuel to drain back after stopping - trying to re-start 2 or 3 times would "re-prime" and be back to normal. Check the exhaust
on your start attempts - no smoke at all = no fuel.
2nd most common was a lack of voltage - as mentioned, the older ones draw 20amps on startup, and there is internal voltage checks on startup - my original wiring
harness was obviously not allowing enough juice. I ran a parallel 12V+ from the fuse to the heater connection and that fixed the problem.
Let me know if you need any more "hints"