It probably saves money to share. Most places like that aren’t on timers. When I was living out of my car at the end of 2017, I used one of those showers for $15. If one traveler uses it, and has another traveler stand outside the door, they could probably switch places really quickly and split the cost of one shower. I don’t think they’re timed.
You can ask for a team shower, in which case you each get separate showers for only one credit on your loyalty card, since you are taking turns driving the same truck.
Or you can ask any other drivers if they have a spare credit, since OTR drivers normally end up with extra or unlimited for the month, depending on the chain. I used to use one just to shit in a private room sometimes.