dirtymudder
Welcome to the forum. I for one understand your concerns to gain a slight bit more milaege during a long trip like that,and other trips in the future.Even if a 1/2 mile per gallon is gained the savings will accumulate so instead of thinner oils lets talk about slicker oils.
You are a prime candidate to use the Schaeffers 15/40 synthetic blend.Time after time this oil proves it's value through engine oil analysis. It will take the heat created from mountain passes and will get you there and back without oil change in between.. I assume the trans uses a Dexron III fluid. Schaeffers type III fluid is as good as it gets and they also have the 131 Nuetra which will help lube the upper cylinder area of the canted valve engine you have that with the 1.7 rocker ratio that wears the valve guides out a bit quicker than some other engines.
Also the # 267 Scaeffers gear oil would be a good choice here. These are not OTC products but are OTP with free shipping and a 800 number.
There are a number of other OTC products to get the job done but I sincerely would take this route if it were my motorhome. Please take time to look at the home page of site supporters and find the Scheaffers Rep in your area. You can call and discuss these products and will get shot straight because David and Bob are commendable and trust worthy Men
If it were a newer engine I would look into a synthetic but I have not personally seen evidence that any of them will outperform the 15/40 blend in terms of pricing and performance so if you are concerned bout leaks,the blend I doubt will let you down there
If I had to use a OTC oil in my motor home
Mobil 1300
Chevron Delo 400
Trans fluid would be
Pennzoil
Chevron
Fuel additives
Techron- but is does not lubricate as far as I know.
I hope this in some way helps and please don't let any pokes and jabs at your wants and needs concerning trying to mke your engine more efficient keep you from coming back to the forum,,I can appreciate what you are looking for and bet many others can as well