OK, guess it is time to answer the huge question that was asked, WHY!
We had a fifth wheel for over 10 years before switching to the motorhome. We had issues with the sensors in the fifth wheel where they would read full even when the tank was empty. We want to make sure that the sensors remain clear of “stuff” so that we do know exactly how full they are.
According to Murphy, a guy we totally trust, if we clean the tanks it will be a total waste of time, but, if we don’t clean them they will get permanently coated with stuff and become useless.
While camping we generally leave out tank valves closed and open them only when a tank gets more than 2/3 full.
We leave Thursday for the VA Samboree and will be on the road for 4 hours getting there and want to use that time to basically wash the sensors to insure they continue to read properly. After reading this thread we plan to make a nice squirt of Dawn in the John and also the kitchen sink, add 5 gallons of water to each, drive for 4 hours, then empty the tanks when we get to VA.
Thanks to all for the advice!