Sorry, I am slow to respond to this, but I don't normally hit this forum.
I use DI water all of the time. My average bill for DI water is about $750 a month, and I get a really good price.
It depends on what you mean by spotting. Is spots or is it streaks? are using soap and if so what kind.
Some soaps just do not rinse clean, even though they claim that they do. You might need to change soaps. Another problem is that people rinse too fast. You might want to slow down just a little and let the water run over the vehicle. Make sure that you start from the top and work down. Otherwise, the water from the top will leave marks on the truck, car or what have you.
It could also depend on your water source. If there is new construction in the area, or pipeline construction it will affect your water quality. Sometimes, if the water is really bad in one area, the cars will spot up and in another area, they will come out spot free.
IF your water is stored in a water tank, I have heard that the water will lose the ionic charge after awhile and you will get spots. Also if the water in the tank is not DI water it could cause problems.
Finally, it is also possible that it is too hot, and causing problems. It does not make sense, but it appears that it happens from time to time. I think, but I am not sure, that it is the wax on the vehicles that is soft, and the soap loosens more and then it spots up and looks like a calcium deposit.