The toilets were installed too close to the walls. The toilet tanks are right against the wall and there hasn't been a problem. However, now one of the tanks has started to do a slow drip. Is there an offset toilet with a tank or something I can do other than chipping out the slab ?

There are offset flanges available. Find Closet Flanges and other Toilet Repair at Aubuchon Hardware

What is the measurement from the finished wall to the center of the toilet bowl caps at base. Do not measure from the baseboard... measure from the face of the wall...

A 12" toilet is a normal rough...sounds like you may be tight....say 11" or less??

BUT... they do make a 10" rough toilet...not lots of options in terms of looks, but can definitely order a 10" rough toilet... they even work up to 9.5 inch installations... but hopefully that is not your situation.

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You probably have a condensation problem causing the drip. Now since it is winter the water entering the tank is very very cold. This cold water then cools down the ceramic tank causing the humidity of the bathroom to condense on the tank. You can try to use one of the tank liners available to help with the cold water problem causing the drip.

You may want to monitor the humidity of the bathroom and use an exhaust fan after bathing. There could also be an air leak to the outside at the area where the floor meets the wall behind the toilet that needs sealed. It's the outside humid air also that is causing the problem. Also check the outside of the wall area to see if there are any leaks outside that need sealed. I had the same problem with my tank leaking and solved it by sealing up all the outside air leaks from cracks. Now it does not "leak" any more.

You say that the tank is too close to the wall, is it the same at the top of the tank that it is at the bottom? The reason that I am asking is that you can take the tank off the bowl and set the tank back onto the bowl with 2 pieces of sponge rubber about 3/4 inch thick on the back edge of the bowl on either side of the main hole. Then when you tighten up the bolts this will make the tank lean a bit to the front of the commode. The gasket that you use between the tank and bowl will be soft enough it will still squeeze down and seal between the tank and bowl. I hope this works for you.