Some quick thoughts:
You can calculate a net shear force parallel to the foundation and a net overturning moment on the tank and on the foundation due to the uneven loading, and the results of those will be similar to seismic loading, only as sustained loads.
Once you have that uneven loading, that may promote even more settlement.
A fixed roof might not drain right.
Manways on the down side would be inaccessible.
The tank could float when empty due to rainwater ponding around the down side.
Water ponding around the low part will lead to repad corrosion.
Leak detection tubes may be inaccessible.
Stairs or ladders could be out of plumb, or leaning backwards/outwards in hazardous ways.
Roof columns would have gravity lateral loads, I assume a minor issue, though.
You normally derive hoop tension assuming an axisymmetric loading. I don't know what happens with asymmetric loading. Could you get big buckles in the shell? You'll at least have some big shear loads that are not normally generated.
Bottom may no longer drain to the sump, and could trap several feet of product when the tank is "emptied".
A floating roof will float level, but the hole it's in will now be elliptical.
Capacity is reduced. Gauging tables are useless.
Instrumentation on the uphill side won't show depth on the downhill side and could lead to overfills.

All good thoughts. The design basis is liquefaction induced settlement and all we care about is tank integrity. Operational issues are not considered. This is at this time just a survivability study for a 500 year seismic event, Ignoring slosh for the time being...

In that event I think the biggest issue is ovalisation of the tank, especially the higher rings which are significantly thinner than the ones at the bottom.

Once the tank starts to bend away from a perfect circle, the forces will increase as the tank bends, meaning more bending which means more force....

That a big tank with up to 18000m3 of liquid in it. That's a lot of force even at a small angle.

How you calculate it I don't know and maybe what all it would need to "survive" is a more strengthening rings / girders than you would normally fit to keep the round tank perfectrly round.

that's not to say that the issue over foundation anchors and the tank sliding sideways needs to be ignored or the moment forces on the foundation, but again maybe just a lot more bolts and strengthening of the wall / floor plate will mean that the tank "survives"

All in all I would suggest you either remove the liquefaction potential material or pile down to something solid.

Prevention is much better than "cure"

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

I would try to rectify the situation before erecting the tank. From a plan view, the tank outline will seem to be elliptical in shape, however, when the roof is welded to the shell, I would expect to see some portion of the shell buckling slightly during fit up. An updated version of my 2nd edition of the J.F Lincoln Arc Welding "theory and design of Tubular Steel Structure" which has a whole chapter devoted to buckling of cylindrical shapes would be one of the references that should help you.