The problem is the shut off valve on the shower head. I have had several trailers and the ones that go cold when you turn the shower wand back on are the ones where the valve passes. When the valves shuts off tightly, I've not had a problem. My theory is that when you are using the shower, you have balanced the hot and cold flows at the shower taps to get the req'd temperature. Now when you operate the shutoff valve at the shower head, and it passes a small flow, then the throttling effect of the taps is negligible and another throttle takes effect. This is the throttling at the hot water tank outlet check valve. This restricts hot water flow and so the cold water wins. The longer you take to "soap up", the more cold water enters the shower hose and wand.