50. Relax, you aren't going to get good MPG regardless, so if you want something, take it (assuming you bough a capable truck that isn't overloaded with an empty RV). The nice thing about an RV over tenting is you leave all the camping gear in the rig and just hook up and go. When you decide you want something, it's in the RV not back in the garage.
The easiest and probably most effective thing is to run with the tanks empty. That will eliminate a lot of weight (as in hundreds of pounds). The exception being boondocking take enough fresh water but consider if you can fill up close to the destination.