Let me re-phrase that, how on earth are you going to save gas by towing a camper around? By changing a chip? I know that a chip can do more than just change shift points. It can also change air to fuel mixture and firing points, but lets get real here, if a chip get you from 10 MPG to 12 MPG while you are towing without sacrificing performance, Lexus would have put that chip in there from the factory. Lets look at what you are trying to move here. The GX itself is almost 5000 pounds and add your camper on top of that, you are talking about around 10000 pounds. Realistically, how many miles per gallon do you think you can improve by? If you want to save gas, dump the GX and the camper and go buy a Prius.
At the risk of beating a dead horse, I bought the GX to tow a camper, but don't tow it all or even most of the time, so am willing to sacrifice performance when not pulling the camper in order to gain fuel economy. I'm aware that I could dump the GX and camper and get a Prius, but my question wasn't how to change vehicles to gain fuel economy, but about the possiblity of altering the GX to acheive that end. Once again, I must apologize for not stating clearly enough that I wanted to keep the GX and the camper, and just wanted to improve mileage when not towing.