Our gas mileage varies from 6.5 mpg to 9 mpg when there is no wind. Add wind and gas mileage will change significantly. Driving locally, 4 - 5 mpg is the norm. We have a 32' Class C with 2 slides. I like to drive within a couple miles of the speed limit when on the interstate. Around here, that is 70 mph.
Most likely the largest holding tank you will find on a Class C is 35 - 40 gallons. Our holding tanks are between 32 and 36 gallons each. I believe the fresh and the grey are 36 gallons while the black is 32 gallons.
We spend the night in a Walmart a few times a year. Generally we do it to either get a head start on travel (leave home a 8 or 9 pm, pull into a Walmart and then get going early in the morning) or when we are traveling long distance and just need a break for 6 to 8 hours to get some sleep. We do put out our bedroom slide if there is room and will run the generator. It is rare to find a Walmart along the east coast that doesn't have Tractor Trailers idling all night, which is about as loud as our generator. If the Tractor Trailer has a refer, it'll be louder than our generator.
I agree with SDCampOwnerOperator, if your objective is to see the country, doing so from a Walmart or Truck stop is not ideal. West of the Mississippi, very low cost and free camping options are plentiful. You will have to do a little more leg work to find low cost or free camping along the east coast, but it is there and it will be a nicer stay than a Walmart or Truck Stop.
When looking at 10 year old motorhomes, make sure they have a decent amount of miles on the odometer. A low mileage, 10 year old RV probably means it has been sitting in one place a lot. If it sat too long, you could have mechanical issues with seals and gaskets blowing out. Even if the tire tread looks good, check the manufactured date stamped on the side wall. You should consider replacing any tire that is more than 5 years old.
-Michael