Travel with fresh tank full, trailer is more stabile with lower C.G.
Black tank.... start with several gal of water and your chemical of choice.
Do not leave Black Tank valve open when using full hook-up, empty only when near full. Black tank first then grey to partially flush the stuff.
Disposable nitrile gloves.
If no line at the dump station and good water pressure I will close the BT valve and use the on-board flush & re-dump.
I do not put chemicals in the tank w/o adding at least 3gal of water, a 1/4 cup of Dawn once in awhile seems to help also.
I did do this procedure
three years ago, haven't had to repeat it since.
Take your time...even with others in line. A mistake could make it very unpleasant for those 'behind' you.
If you have a convenient station close to home, I've found that traveling with a partially full black tank helps a lot in preventing "black tank constipation".