Well it took a while to get going but the tank replacement is complete. It is over a week now and after 2 flushes no leak so hoping for the best.
The work took about 5 hours. The biggest problem was to take the old tank out. It sits on the frame with the floor on top of it. After cutting the toilet pipe and vent pipe flush with the floor you still have 3/4 inch of pipe left which prevents you from sliding the tank out. So I used a gig saw to cut away the pipes then I could slide it out. These are NOT set up for easy replacement.
My concern is that the black and gray tank sit side by side, the valves are connected directly onto the tanks with a Y in between. This does not allow for the use of flex couplings, nor the use of ABS glue to secure the valve into the black tank. Could only glue it on the outside.
Here are some pictures.
The old and leaking tank (look at the internabond tape):
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Cutting away old tank connections, sealing old tank.
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Preparing new tank, connecting new tank using flex couplings
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The finished product:
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Hope these pictures give you a better look at what is involved when changing a tank.
If you have any questions I will gladly answer them.