We have the black tank flush and have used it. Trouble is, it seems to have a bit of a leak somewhere - when we use it, water comes out from underneath the camper (!) Haven't noticed any huge problems with it or leaking into the camper, so we just don't use it very often.
My guess re: the antifreeze hookup is that it's for winterizing the water system. Haven't ever heard of doing it from the outside before. Just in case you're a newbie with RVs, do NOT use car antifreeze (blue) for this! :E
Re: glass stove top - interesting. You might want to put a towel under and over it for transit. We do that with the (glass) turntable in our camper's microwave.
Incidentally, Rockwood & Flagstaff are twins, and made in the same factory. You know it's one or the other at the end of the build when the decals go on. :)