Too bad you can't figure out a way to install the new one and keep the old one. Any way you could install it under the frame and then install a cross-over from one tank to the other? With the cross over, the original sensors on the original tank would still read as it always had, it would just be a lot more water?
You would need to vent the new tank, and of course have a drain. But both tanks could be filled from the original fill location. The cross over would fill the second tank. (It's just an idea if you haven't thought of this yet. But, I don't know the configuration of your camper either.)
Good luck!
