Not necessary to "fill" the tanks completely to rinse, 5-10 gallons should be sufficient.
A clear section should be added at the rv end to be able to check when it's running clear.
ALWAYS connect the sewer end of the hose securely 1st gen while holding the rv end as close as possible remove the cap from the rv to try a run the dab that comes will go down the hose.
If you get a cap with the water hose fitting (see below) on it remove that small cap 1st, all the valves tend to leak a bit & there could be several feet of piping that doesn't completely drain that WILL run out 1st.
As mentioned the tank chemicals are not needed, once rinsed & flushed add a couple gallons of water to the black tank then add a good healthy squirt of cheapo dish/laundry detergent, this will work to keep valves lubricated & work as well as the expensive tank chemicals.