Finally did some mostly highway driving in the truck, and we got 16 mpg. We were driving 70 for much of it, a speed we will avoid when pulling the fiver unless traffic pushes us, so I could have done even better. Based on the experience of others, we should be able to do 14 mpg towing. If the propane injection does as some suggest, maybe even better.
All in all this means our actual fuel use should be about 30% lower than we have been calculating as we budget – sweet!