Let's start with AquaHot basics. Depending on model, the AH has two or three ways to heat the water used for heating and domestic hot water:
a. Diesel burner
b. Electric element
c. Engine loop (sometimes called "motoraid")
There should be two or three controls for the AH, depending on whether you have engine preheat:
3. Engine preheat
They might be labeled differently depending on which generation of switch cover you have. If (1) is off, the AH should not burn any diesel. You should not hear it start or run.
If (1) is on and (2) is on, the electric element might or might not keep the AH above the set temperature for the diesel burner. If it doesn't, the burner will start once in a while to bring it back up to temperature.
So the first question is whether switch (1) is on. If it is, you might be smelling diesel exhaust from the AH. You should be able to hear it run, if so. Is it on, and running, when you smell diesel?
The second question (I suppose it should have been first) is whether you are smelling diesel fuel, or diesel exhaust.
1976 Southwind 28', '96 Winnie 34WK,
2006 Tiffin Allegro Bus 40QDP