In our 9 years of using our 36TK3, we have always kept the black tank closed--period--except when dumping the tank. If you don't let some fluid build up in the tank, you will have solids build up and end up with a mess that has dried and hard to get moving. I would keep it closed until at least 2/3 to 3/4 full before dumping and flushing.
As to the gray water tank--much discussion on that: 1) keep it closed so you will have some non-black water to flush sewer hose after dumping the black tank; 2) leave it open and forget about the tank getting too full, use the flush device to clean the sewer hose after dumping the black tank.
A lot of this will come to you thru experience, hopefully a not too smelly experience. If you move often, the movement of the fluid will help keep solids from settling/building up. If you are stationary for long periods, that is when you will have to get a routine established that works for you.
2016 Tiffin 40 QBH
2015 38RSSA, traded
2005 TK3 #1869, 10 yrs of memories,
2017 F450 KR--one more Ford is it
2009 F450 4x4-died; 2010 F450-retired