Cutoff for Shower Head? (2023 400)

For a variety of reasons, we prefer to shower at "brick and mortar" facilities--in other words: outside the camper.  Our aesthetically pleasing shower in the 400 mostly serves no purpose, except to occasionally have the mixer get unwittingly bumped by an elbow (when brushing teeth, say), which leads to--at best--a very slow drip and unexpectedly wet feet later on, or--at worst--a sudden blast of water and cursing until it's shut off.  I'm wondering if there's a simple, no-cost solution to this problem, like an easily accessible shutoff valve somewhere?  Alternatively, perhaps someone knows of a bit of plumbing kit that could provide a cutoff along the shower "hose" line instead?  If it's the latter, perhaps someone who's done some previous plumbing work in their Tab knows what the fitting dimensions are?