I store things on and under my back seat from time to time. I imagine the seat is approximately 4-5 inches compressed.
I don't find using it that way a problem since you can literally put things from the top of the seat to the roof if needed since you can't see out of the back anyway. I also have one of the Du'Ha underseat storage compartments.
http://du-ha.com/index.html My dog crates go on top of the seat. The advantage of this approach is that it isn't obvious that you have anything under the seat.
Obviously, if you took out the seat, you couldn't carry passengers at any point without reinstalling the seat. Since I use my truck without a camper, permanently removing the seat isn't a good idea. In bad weather, we typically use my truck to travel even without the camper.