I did just what you want to do in my 1980
Trillium Jubilee although I had just about gutted the complete interior so had more or less a blank slate to start with.
I was able to rebuild the kitchen and carry the new countertop around the back of the seat on that side of the trailer. This seat was made not so deep as the one on the opposite side as my wife didn't need as much room as me and this is what enabled me to make storage area behind that seat. I also took the corners off of the table that I built to match the countertop.
There was a lot of mocking up done with cardboard, bits of ply, sitting in time and time again to get the optimum sizing for the space available.
The compartment under the seat on the door side of the trailer houses a Porta Potti and all of it's ancillary stuff.
The compartment under the other seat house the
electrical stuff, Converter, Inverter, 3 Way Switch etc.
I have many more pics if you want anything specific will look to see if I've got anything pertinent.
Good luck with your project ... don't plump for the first thing that comes to mind. Try something, mock it up and consider it's effectiveness.