When I bought my 5th wheel last year the dealer didn't tell me (when I asked it appeared they didn't know) about the second set of release handles for my two rear tanks. I used a paint pen on the frame to indicate which was black and gray and also used the paint pen to mark in the plumbing compartment which handle controlled what tank. I think the paint pens are better for something you want to permanently mark. Also "Grey tank" written next to the handle on the wall means I wouldn't have to worry about the grey paint on the handle coming off and then I have two black handles again.
2013 Brookstone 340LS Ruby
2014 Dodge 3500 Diesel