I use oxiclean free and hot water if it needs a soak, otherwise the mash tun just gets rinsed with the hose. My BK gets an oxiclean recirc and quick scrub with a hard bristle brush, then I rinse with the hose.
What is your efficiency looking like? What kind of cooler do you use?
HERE is a link to some descriptions of off-flavors and their causes, I've never heard of milky but watery could be a volume issue or efficiency issue. I'd probably look somewhere else for the off-flavor, I doubt the mash tun is to blame.