Two weeks ago I upgraded our 25 gallon grey tank to a 55 gallon grey tank.
The project went as expected. The existing grey tank uses one removable angle iron bracket and the other side of the tank sits on top of an existing cross member. I had a local shop fabricate a second bracket that was needed due to the dimensions of the new tank. All in all it was about a $600 cost between the tank, bracket, fittings, etc.
Been camping the last two weeks. Did 5 nights in Sequoia National Park last week and didn't have to dump the tanks.
I was able to route the new dump valves in the same position, and everything is working great so far. Being that the grey tank was and is behind the rear axles I wouldn't tow with it full given the size and weight shift involved.
I will try to add some photos. All in all, it went as expected, but is definitely not a easy switch out. It is a complete custom job, and takes a good deal of research to see what size and shape tanks would work.
Sorry about the photo alignment. If anyone wants more info or additional pics PM me. The bottom photo is the old tank.
Here is a short video showing the final product before closing it up: