Fix Weak Shower Pressure

1 Toilet Flush = 1 Full Motorhome Shower

Do I have any plumbers following me? Or people who are great at DIY plumbing stuff?
I’ve got an issue with my shower, absolutely no spoons to deal with it and the worst, strangest shower fixture I’ve ever seen. Maybe it’s more common in other countries?
It’s a Riobel thermostatic and the little handle that controls temperature is NOT working.
I’ve been trying to tolerate it but got scalded badly tonight and I’ve had enough.
If anyone can help me kick ideas around I would be super grateful.