As for water, it all depends. How long will we be gone and will we be in territory we know.
If we will be gone a long time, I fill the water tank. I like having water we are used to available, if we need it. I also fill the tank if we will be traveling with many folks.
If it's just the wife and me and we are traveling a short distance (Florida Winter camping with friends) 1/4 tank is fine.
I have 105 gallons of freash water capacity. At 8.3 lbs/gallon it comes to 872 lbs. I have never been able to determine the cost (fuel) to carry the additional weight. At this time, I feel the weight is an insignificant hit to the MPG. The items mentioned in my OP make the big differences.
My toad weighs 5074 lbs street weight. The best I can determine the toad costs me .3 MPG. Consider carrying the amount of water you are comfortable with and forget the effect on MPG.
On the topic of A/C, what has been previously posted is also my experience. Consider keeping the coach comfortable for everyone onboard. My gen set takes about 2/3 gallon per hour. It is not enough for me to be concerned about.
2005 Newmar KSDP 3910 + GMC ENVOY XUV 37K lbs Moving Down The Road
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