Your question about the gray water filling the black tank. They are 2 different systems and cannot fill each other, at least in all rvs I have had. They flow thru the same drain pipe at the last tee were the dump valve is located. Before that point they should not be any cross contact.
2013 24RKSWE (27ft TT) Cougar 1/2 ton series
2013 Ford F350 4x4 CC 6.7 engine, 8 ft bed 3.55 rear end lariat package
Retired from Oregon State Police in 2011 than worked another 9.5 years as a small town traffic cop:
As of 05-2020, I am all done with 39 years total police work. No more uniforms for me.