Typical RV water system......

Fresh water tank & pump OR 'city water' connection on side of rig supply the hot/cold water plumbing (sinks/showers/toilet)
Make sure that water heater is full BEFORE using the heating source(s)....propane/electric....ESPECIALLY the electric (it will burn out if not submersed in water.
Water heater will fill when running pump OR turning 'city water' supply on...just open closest faucet (hot side) to get air out of system.
Black tank (toilet)......best to dump when full/almost full (high liquid volume & weight carry the 'stuff' out.
After dumping....flush/rinse tank if possible. Then close drain valve and add a couple gallons water back into black tank (two toilet bowls works). This will keep bottom of tank wet so 'stuff' doesn't stick to bottom, dry out, pile up & stink.
Then drain grey tank....soapy water helps flush/rinse drain line and sewer hose
Have fun.....go camping.