I'm doing a similar thing part-time on a much smaller scale. My business requires site work, so I'm more limited in how much time I can stay away. Plus I'm certainly not making "millions". But I am financially viable with what I do (and my van certainly does have a wet bath).
I will say that it requires breaking a lot of paradigms and getting very good at task-shedding, because the distractions are endless and enormous. And it also requires getting imprinted on the van itself, if that makes any sense. The thought has to get well-entrenched that the van is where work gets done on a routine, flow-structured basis. In other words, it is not sufficient to note to oneself, "I'm working in the van now instead of in the office". The headspace has to be more expansive than that. The van IS the office, period. The van is where I make paying projects happen.
Here's a pic of my workday yesterday, view to my left as I was doing client work while my dog patiently watched the world go by, waiting for me to finish.