The "blast of cold water" has been discussed here before and on other forums. The solution is very simple. Add a check valve to the hot water supply line of the shower and a "complete" shut off valve that stops the slow leak of the shower head.
A nice video I found of someone modding their RV:
https://www.thefitrv.com/rv-tips/how...our-rv-shower/
A previous forum discussion of a fix:
http://www.keystoneforums.com/forums...ad.php?t=25493