There are certain variables for the answer:
Size of the tank. Someone with a 100gal tank may only do a partial fill, while someone with a 36gal tank will do a full fill.
Type of use. Those going to full hookup sites are likely to just maintain a minimal amount in the tank for "while on the road use", while those dry camping are more likely to fill it.
Frequency of usage. Those who use their RV on a frequent basis year round are more likely to not drain than those who store it for a season.
Location during non-use. Those who store their RVs off-site are more likely to drain and/or not fill completely than those who store at home.
I had the privilege of talking to a sewer and water treatment employee for a park system. He showed me photos of the parks' potable water tanks. I'm not inclined to fill away from home. I know my water is clean and safe - I don't know what the water is like somewhere else.
I have a 40gal tank and I do not see a difference in mpg when it's full or empty.
As I also live in an area subject to drought and water restrictions, I would prefer folks coming from regions with no water shortages to bring their own water. Just seems to make sense to get water where it's plentiful rather than wait to fill in a place where the public restrooms are closed because there isn't enough water to operate them.
