Highly doubt dealer went to the trouble of flushing fresh water tank.
Should be a tank drain on bottom of RV.
Sanitize tank/plumbing lines with household bleach (1/4 cup per 15 gallons of water)
Add appropriate amount of bleach to fresh water tank....turn on water pump and open faucets (hot/cold) one at a time until water flows and you can smell the bleach (Kitchen sink/bathroom sink/shower faucets and toilet. Then let some water out of low point drains (drains for hot/cold plumbing lines).
Leave over night........then drain fresh water tank, drain water heater, fill/flush tank. Then fill tank.......flush hot/cold lines/low point drains to get rid of bleach water.
Little baking soda added to fresh water tank helps