Since we really didn't know what we were doing as far as plumbing, tanks, etc. we figured one tank would work ok. Our fresh tank is about 120 gallons and we have a gate valve between the shower and single waste tank (60 gallons). At alot of the race tracks its ok to drop on the ground and it really helps to just put some of the showers in the ground!
We do usually camp in the trailer for a weekend and haven't had any problems with running low on fresh or filling up the waste tank. As for taking 10 minutes at the dump station, we can't get into any of the ones I've seen; we have a septic tank that we pump into where we store our trailer.
We do sprint car racing, mostly in the upper midwest. I found one picture of the car where you can kind of see the cabinets inside the trailer. I didn't do a really good job documenting the the build, but will have to see if I can find some pics to upload. I know this is a truck conversion forum, but figured there might be some interest. I basically consider part of the trailer a conversion and as I said earlier, really used this board as a good resource