If you are dry camping and have no external water supply, drain the black tank first and then the gray tank. The gray water will sorta clean the hose. You can do a thorough cleaning job when you are at a dump station with water. Without an external water supply you can't really do a great job cleaning. If you do have an external source of water just fill up your tanks and dump black then gray.
As far as the sewer solution goes, I have one but I only use it when at home as I am 30' or so from a drain. At a dump station I use the 3" accordion hose. My thinking is that there is much more pressure pushing the stuff out of the tank with the larger hose. The Sewer Solution breaks up the solids so they will make it through a 1 1/2" hose. There is basically no pressure.
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