I cannot imagine why anyone with a diesel fired Aqua-Hot would ever want to convert to propane. The convenience of using diesel is just too great. And no, a properly maintained diesel burner does not smell or smoke at all when operating.
When hooked up, we have both our diesel and electric switches on 24/7. When heating the interior of the coach, the diesel burner will run for 5min, 2-3 times per hour. When no coach heat is required, the electric element will maintain hot water except during a long shower, so our diesel burner runs maybe once or twice per day, for 5min or so. Like everything else in a coach, I believe using it regularly is beneficial, compared to letting things sit unused for months. At 1/3gal of diesel per hour of operation, I'll not worry about 10min of diesel burner run-time per day.
Be warned though.... Once you have a coach with an Aqua-Hot, you will NEVER want to be without one! ;-)