That is fairly normal.. Most RV showere when you RESTORE flow after cutting it off you either get blasted with HOT or COLD. The solution...
Oxygenics shower heads come with what they call a "Smart Pulse" cut off valve. it allows a small "Dribble" of water when in the cut off position.. Seems to prevent this blast issue.. THe shower Cut Off Valve I have also does the same thing.. Exactly. with the same result (it was sold as a shower cut off valve.. The old valve was a quarter turn valve that fully shut off the flow. And I had the blast issue when restored. But the dribble valve does not.. I'm thinking the smallest drill bit you got through the ball of the valve might fix it. I mean the smallest (i'm an electronics tec so my smallest is ... SMALL )