For me, I would prefer to have the security of a steady base as opposed to only trying to focus on upon new business. Offering up customers, after the initial detail, a the monthly maintenance program of 3 weekly washes and a wash/wax will help accomplish that. Since the vehicles are in good shape, and in most cases just high pressure rinsing and drying will cover the wash, the washes take about 15-20 minutes. Using the same reasoning, wax's take about 30-45 minutes. This not only opens the door for being at each location on a weekly basis helping to expand clientele, it also helps get through the winter months. If you can have them pay in advance, it also helps with a steady monthly income. As an incentive, one could also offer up those who sign up a discount on other detailing services.
Sign up 10 of these per day, 5 days per week, at $100/ month that's what $5k? Add to that the other detailing services that can be preformed in the remainder of each day, and add a complete detail at least once a year for each of these 50 monthly clients, and that puts one's mobile unit into the $75K/yr range. With two people, in a major metro area, one could easily push gross sales into $200k/yr bracket.
These are just numbers used as examples though... many will make more.
I might add that in the area of not being able to find the right helper/employee that there is a little wisdom in the sayings.... "wherever I go, there I am..." or " I take me, with me, wherever I go..." For me, if I am the common denominator in my issues/problems, then I might want to look into me, what I am doing wrong, and maybe try to fix it.