Yeah ,Tug is right!!! I have used Greased lightening and let it soak, then go to a car wash. After you are done, it is a good idea to rinse down the car wash out of respect, or else they may hang up signs " NO MUD OR GREASE CLEANING" . If I am doing a rebuild I will rent a steam cleaner like Tug is talking about, it reallly is fast and cheap. The steam vapor blows away the grease and some of the paint too.
If you do use a steam cleaner , or any high pressure washing unit, car wash etc. , wear safety glasses , and maybe a rain coat or poncho. When the stream comes back at you sometimes it can blow debris and soap in your eyes at very high pressure.
The really best way is rent the steam cleaner, use a pump up sprayer like you see in the garden department to spray the detergent . Get a rain poncho and lay down where you can see to spray all under your rig.
" While wearing a face shield". Let it soak. Then steam away all of the grime.
I have heard of people using the water from the drain on their hot water heater????????? I would be afraid it might not shut off good and disturb the rust and calcium in the tank and cause other problems.
Later,
JAckal