I found this early in our truck camper experience. I don't know who wrote it to give credit but I modified it a bit to add flushing the tank via the petcock.
1) Open low point drains (blue and red caps on bottom of camper below water heater) to drain some antifreeze. Reinstall caps.
2) Fill fresh water tank with clean water.
3) Open shower hot and cold water valves (if equipped) and sink faucet all the way open. NOTE - If faucet only has one knob for hot/cold water, ensure to cycle valve from hot to cold during flushing.
4) Turn on water pump to flush antifreeze out with clean water from the water tank. Observe exit points (shower & sink faucet) until no antifreeze is visible. NOTE - Recommend flushing for 5 minutes or so.
5) Once the lines are clean, fill tank again and locate the petcock next to your water tank and drain the tank through the petcock to clean the remaining antifreeze from the tank...repeat. Water will drain through the bottom of the camper. Its unnecessary to run all the water through the water pump to clean the tank, its only necessary to clean the water lines to the kitchen. The rest can drain through the petcock at the tank.
6) Fill fresh water tank with clean water and add bleach (1/4 cup of bleach for every 16 gallons of water in your freshwater tank).
7) With water pump on, open faucet and shower until bleach is present. Close faucet and shower valves. Let sit for 12 hours or so and then re-flush the system following steps 1 through 5.