I believe I posted a rant on this very topic a couple of years ago.
It depends upon your model, but I did not find it very difficult to replace the tiny double sink in our '86 25' Sovereign
. We went with a 25" single bowl stainless utility or laundry sink. It is not as wide, which gave us back some counter space, but it is deeper front to back. We can now easily wash our largest plates and pans.
There was no trouble with the drain. Ours is pvc, and cutting and gluing it to fit the new sink was simple. As far a losing space under the sink, in our coach the top couple of inches was not usable anyway, so nothing was lost there.
We also replaced the drop in cooktop/oven with a smaller two burner cooktop and compartment for a microwave, plus a badly needed utensil drawer. The gain in counterspace was a major improvemant in making the galley usable.
I suppose my pictures of all this are still posted in the photos section. And why Airstream does not build the 25' Classic this way is still a mystery to me.