Here is a piece of advice that I am not charging for... ..... so it maybe worth what you are paying for it... ... ... .
It is better NOT to leave your black tank open(keep it closed) when you are connected to sewer. When you do, the liquid runs out and leaves the solid behind never giving the potti-chem time to desolve the solids.
Your black system can get one heck of a case of constipation by leaving the black tank open while hooked to sewer.
You can leave your gray water open until the day before you are going to leave. Then close your gray water trapping all that nice soapy water from your sink and shower. Right before you unhook the sewer, OPEN the black water valve letting out all the sewage. When it is empty or after you have flushed the black tank(if you have an on board internal sewer spray), close the black valve and open the gray water valve releasing all that nice soapy water thereby flushing and rinsing your sewer hose... ... ... ...
mi dos centavos... ... ... ... ... .