I have an insinkerator insta-hot water dispenser in which the water has
gotten a funky smell to it.

The filter has been replaced, but the smell is clearly coming from the
reservoir. I disconnected everything and drain the reservoir (in which
the water definitely smells bad). What's the best product to clean
this thing out? My water has a lot of minerals and a fair amount of
rust in it, which was evident my examining the reservoir. So I can
pour stuff into this thing, but what will dissolve all the crap that's
in it?