Does your trailer have a sticker on it with recommended tire pressures? For me, whatever the sticker says is my minimum. Whatever the tire max. pressure on the sidewall says is the maximum. Those numbers might be the same, or they might not. That may help and it may not, but I hope it makes sense.
As far as MPG, have you seen the document put out by Cummins? It is for semi-trucks, but I think the research, findings and conclusions are applicable for RVers. The rock solid rules are on page 4, and the rest of the guide goes more in depth on all topics. Enjoy! https://welove.com/wp-content/upload...el-Economy.pdf