drain the waterheater,
cap off the in and out fitttings
install jumper hose from the lines you took off the heater(your rv dealer has this jumper)
my 20 gal fresh water tank took 2 gals rv anti freeze to flush the sink and outside shower lines left a little in the drain from the sink
black and grey water holding tanks I don't have
there are plenty of rv sites if you search rv winterization..