A lot will disagree with me but I want to be able to legally carry:
a) full water tank
b) full fuel tank
c) full gray tank
d) full black tank
e) full propane tank
f) tool box
g) refrigerator full of food
h) clothes in the closets and drawers
i) both me and the wife
j) computer and printer
k) supply of drinks(wine and beer)
l) junk food or comfort food as my DW calls it
and anything else you need to make your travels comfortable. All of these make up your carrying capacity and as long as you don't go over your maximum GVW on any one wheel you are good to go.