After two years the plastic Hydro-hot drain assembly was leaking on our 2006 36FDDS so I replaced it with metal parts. A number of Hydro-hot owners have already done this; here are some pictures which may help anyone else decide if they want to tackle the job themselves or have a professional do it. Fortunately, the manufacturer switched to metal parts last year, so this is not a problem on the newer units.
I removed the electronics box from the side of the diesel burner for access to the drain.
A short piece of 1/2" PEX tubing fits into the opening of the plastic valve to drain the Hydro-hot tank.
The plastic drain assembly unscrewed easily from the short brass nipple attached to the tank, likely the reason it was leaking. The Aqua-hot factory technician said that the plastic drain assembly could be easily stressed when the diesel burner is serviced.
I took the plastic drain assembly to a hardware store to get the correct size of gas ball valve and garden hose adapter. The gas ball valve screws onto the nipple and the garden hose adapter onto the valve. This metal assembly is much stronger and shorter than the plastic assembly and should not be damaged during routine service of the burner.