Sounds like...
... you've done your homework.I think Steve's point about resale is a good one, but if you have the skill to build in the larger tank so it works as a seat, or refurbish the area when the day comes to sell, that should not be a problem.My concern would be that a prospective buyer would be turned off by a non-standard or odd setup. I know that sort of thing was a concern for me when I was looking, and I saw a lot of do-it-yourself mods that killed the sale. My current setup is a less-than ideal bladder tank under the V berth, installed by the p.o.: good location but really should be a hard tank. I rarely use it, but before I head out for a long trip I would replace it.The composting tanks are new to me, but an interesting idea.Good Luck,Sanders