Since we mainly do the dog show scene when we travel and are at home the rest of the time we haven't had this issue much. However, when we have to bathe someone we use the outside shower of the Moho. We are usually parked on asphalt or dirt so it's not a problem.
The towel is a good idea as it's easy to carry with you and use. Of course if you can do the bath mat it is the best but hard to carry around fulltiming.
The "dry" shampoos are ok for quick cleaning but if you have elderly dogs with incontinence issues or bad skin they won't do the trick.
Inbetween shampoos maybe try using a damp towel and toweling the dogs off. Might help cut down on those bathings.
I know with our elderly mastiff who had those sebaceous cysts that kept opening and oozing--sorry if you are eating breakfast

but we had to bathe her about every day!
Some towns also have "self-bathing dog" facilities that are MUCH less costly than groomers!
Wish we could help you more!