Has anyone added an extra water tank to RV roof

I'd be nervous putting that much weight on the roof. I would try to find somewhere in one of the compartments below and put lawn chairs or something like that on the roof or strapped to the ladder.

I don't have an answer for you.
However, I would be concerned about the additional weight and the impact on the COG.
Are you planning on filling the tank at home and driving to your destination or filling near your destination?
You'll have a little over 400 pounds with the weight of the tank.
But, once you start bouncing down the road the dynamics will change quickly. And momentarily you will have much more than 400 pounds which could be the straw that breaks the camel's back.

It may also affect the handling of your RV.
I would carry 5 gallon jugs inside the RV, you can store them outside
When you get to your destination.

If you decide against trying that, have you considered an Idaho Tote or Freedom Hauler

Nearly 3000 lb capacity so haul all the water you want.
or, a cargo carrier.
cargo carrier
I would keep the weight to maybe 350, but that should still help you, and you could leave it empty most of the time , only filling when your getting close to your destination.
Happy motorin

For short distances from a water source to a camp, I carry four 7 gal Reliance jugs on the rv floor.
To eliminate the lifting, I have a specific clean tub that I pour it into and a specific marine bilge pump that pumps it into the holding tank when needed. I just plug the bilge into my outside solar hookup. That's actually a 2 way 12v power source.

Re the roof: I would rather see that much weight on a front receiver rated at 500# or more if possible. Some of the TC guys carry it that way.

The other option would be an outside portable shower and use a stream as your water for washing .

I also bought from Amazon a portable rechargeable shower head and pump that is submersible in a 5 gal bucket. I have used that inside my shower using stream water. It actually works in the winter when I have winterized my lines. I just drain the gray tank right away.

RV structures are not known for being overbuilt and motorhome suspensions are also iffy.

I would look underneath to see if you could fit something between the frame in an unused area. Also check the weight ratings on the rig. Good chance, you don't have enough cargo capacity to add 500lb.

Do you tow a car? If not a small enclosed trailer might be a good idea.

“Some negatives” include adding 400 pounds plus tank weight to the highest point of the RV. How much did you determine this would raise your COG?

There are several 500 lb rated hitch carriers listed on amazon. Much better solution imho.

Because water is a liquid it does not spread it's weight equal but if there is a point that a little lower or weaker it will put more of it's weight there.

It would affect the center of gravity some, raising it probably a couple or a few inches. Whether or not the roof structure would be safely able to carry the extra weight is a bigger concern; if it's a well-constructed RV, it ought to be able to, as it isn't enormously more than, say, a couple of air conditioners (the weight of which is less evenly distributed), but we all know that many RVs are not especially well-constructed.

If you do install it, be sure the tank is oriented lenghtwise rather than across the RV. If it's crosswise and half full, the water will slosh back and forth from side to side and could definitely cause handling issues or at least annoyances. Weight shift front to back is rather less of a problem--though still may be noticeable. Weight shift can, of course, be avoided by travelling with the tank either full or empty (as opposed to something in between).

All in all, I think a trailer mounted tank is a better idea (a "water buffalo" or something similar/smaller).

I realize there’s some negatives

There is a risk installing a tank below the belly as well.
Many trailer owners have to weld crossmember supports to secure
OEM water tanks installed under TT's. Most water tanks in MH's are resting on solid interior floors.

Is this idea something that you would intend to use while driving on a highway or is it a very short state park campground kind of road you plan to drive on where you don't want to break down your site to go get more water from a spigot ?

On my first search I found a documented case. Most states now have criminal negligence laws regarding DOT and public highways.
In Georgia, A ladder flew off an improperly secured rack mechanism on a privately owned pickup truck and killed another motorist.
The DA made a criminal negligent homicide arrest. The driver was civilly and criminally responsible for 'failure to properly secure a load while operating a motor vehicle on a public highway." He had no defense, he put the ladder there and it was his truck, he was operating it. It was a felony.