What is under the seats? On our previous trailer, the water tank, pump and plumbing took up all the space under one bench. Under the other was the furnace and some duct work and a small storage area. On that trailer, we wouldn't have gained very much floor space.
In our current c-class, most of the space is available except for a fresh water tank in one corner, which could be worked around, maybe an end table. Of course I would have to relocate 12v and 120v wiring and plugs in each endwall.
For resale, there are no rules saying that you have to put it back to factory specs. You may find people would rather have a dinnette or someone that loves your mod.
Ken