I use an Oxford tail pack over the seat, mostly because I use the Velcro strap on the tail pack under the seat before fitting it in place, its a bit less likely to go walkies that way as the seat has to be unlocked to remove it.
It is a large platform without the seat and ideal for tent etc to strap across. I did notice on advrider there is a thread with bikes loaded with their camping gear and nearly all of them had the rear seat removed.
It's great having the option to remove the rear seat. I always intend to remove it when solo so I can pack my gear on it and move the weight more forward but then I end up leaving it in place - you never know when you might need to give someone a lift.....and I find it more difficult getting my leg over with my roll bag on the back seat.
All you need for a 3 week trans US trip if you aren't camping. Roll bag for clothes and top box (which was too big by the way) for spares, tools etc etc.