If you compare the fuel usage of the generator vs the V-10 you get something like:
Generator will use about 1/2 gallon of gas per hour (1/2 load, from here:
https://power.cummins.com/sites/defa.../F-1123-EN.pdf ).
RV going down the road will get about 8 mpg @ 70mph (roughly) which is 8.75 gallons/hour.
If we assume a .5 mpg hit by the A/C then 7.5 mpg @ 70 mph is 9.33 gallons per hour.
Note that there are a lot of assumptions above (especially that I don't think the A/C gives you a .5 mpg hit..I bet its more like .2 or less--something not noticable). Another one is: I don't think the A/C will load the generator up to 1/2 load either.
Doing the math for .2 mpg hit: 8.9 gph.
I'm going to conclude it is a wash: Do either as you like; its not going to change your very large gas bill by enough to be noticeable.
I run with only the front unless its really hot, or a passenger on the couch complains that they are hot.
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