Two different ways when I had my HW 277. If we had hook ups and before we lowered the roof, we would put our rinse wand thru the canvas and in the camode to rinse while draining. If no hookups and roof down, dump the tank first and get a gallon bucket and put under valve. Put rinse wand up the valve and run until bucket fills. Take to toilet and empty bucket. Repeat. Wear gloves.
Valterra A01-0187VP Flexible Tank Wand https://www.amazon.com/dp/B000BGHYE8..._CAhOvb2HJPDK1
Tip: secure the rinse tip by putting a screw thru it. I have had it come off inside the tank more than once. Might also use a few extra holes for water flow.
2014 Stealth Evo 2850- "Woodstock"
2011 Toyota Tundra Rock Crawler TRD 5.7- "Clifford"
2013 Honda Accord Coupe V6 w/Track Pack- "Julia"
Just glad to get away