Mixer valves can be poorly behaved. A little scale or mineral build up in them can cause a would of trouble. I know.
I have a shut-off valve installed at the base of the shower head arm to allow quick on-off while soaping up and rise-off. When I turn the valve back on it would be very cold for a short while. That problem was solved totally by adding a inline check valve on the hot line close to the shower.
Fred & Denise (RVM157) New Mexico
2007 Excel Classic 30RSO & Coach House 272XL E450
2007 RAM 3500, Diesel, 6Spd Auto, SWD, 4x4, CC & LB