Just have to ask because of the time of the year it is. Are you sanitizing or winterizing? There is a big difference in outcomes. I do both jobs while the HTT is sitting in the street (I have no off street parking). Chuck is right on the money about time and water volumes. You can use a higher amount of bleach to sanitize faster, but you need a lot more water to flush the bleach out of the system. I try to let my bleach water sit in the holding tanks and pipes over night. That way I know it has sat in the lines long enough, and then flush the next day. I think flushing is the worst part of the job. I typically only sanitize once a year in the spring when I dewinterize the HTT.
Where to do it, wherever you can get a hose to reach and the TT to sit level enough so it will fill the holding tank and sewer system will drain. I do not like putting bleach down the toilet as it will kill the good bacteria, but I have to do a little just to flush the water line.
You do not note where you are located. Just note that if you live where it freezes you need to drain the system of all fresh water (tanks, lines, P traps, etc), or you will rupture a pipe. If you live in the arctic north you will want to use RV antifreeze, if you live in the south, you can get by with using compressed air and blowing out the lines, but most people still like to use RV antifreeze anyways as it is cheap insurance.
Best of luck
2012 Jayco X23B
2020 Ram Laramie 3500 SRW Air ride 50Gal fuel tank.
2007 Dodge Dakota Quad Cab (retired from towing)
Equal-I-zer 4-Point Sway Control