If you would be going through this expense just to hold enough gray water for a long weekend then all you probably need to do is to instruct your family on conserving water. A weekend shouldn't be any problem. Never leave water running for showers, dishes, washing hands or even brushing teeth. You could also forego a daily shower. Freshen up with a washcloth instead.
No argument there. I've been on dirtbike trips in the boondocks where 5 guys got by on 20 gallons of water for 4 days and backpacking trips where I got by on far less. I like to camp like that but my wife not so much - and being as I don't dirtbike/backpack any more we're using this trailer as a way to get time together.
So roughing it is not really the point. Even though the kids have moved out now we're both still working every day. The idea is to make this trailer a budget quasi-luxury spa experience - a nice little oasis for when we get out of town. Sounds a little goofy I know, and it's not a fancy rig, but the rear lounge plan has two nice chairs for watching movies, I've put in the largest TV that will fit, got a full-size pillow top mattress and fancy sheets and fancy soaps in the bathroom - just like the Ritz (kinda) :).
The Mrs. likes to cook fancy stuff in the little kitchen because she rarely has time during the week. This means great meals and dishes - plenty of dishes. I've scoured thrift stores for the china and glassware which all has to be washed as well. I don't mind the washing as long as she likes the cooking, and we often invite guests.
The larger tank will make for one less thing to worry about (without guests we can go only about three days now without filling up the gray tank) and considering what a similar hotel stay would run the larger tank seems like quite a bargain.
Besides - I like messing with this stuff. Once the tank goes in I am installing a wine fridge. :B