There is a difference between the smell of diesel fuel and diesel exhaust. The poster says diesel fuel. We had a diesel fuel smell with our older AHS-100 (2001 Monaco) unit. It turned out to be a leaky fuel pump inside the webasco unit. Replacing that solved the problem after a good cleaning of the leaked fuel. In talking to others, this is a common problem with older units.
Replacing the pump is not too bad a job and the complete service manual including the procedure is online. The only way to tell for sure is to inspect the pump which involves removing the webasco unit from the combustion chamber (two screws) on my unit.
If you post your aqua hot model number you may get some more specific advice. Good luck.
Bob, Janice & Guinness the Hound
2001 Monaco Executive (MuttRHome)
with Saturn Vue in tow