Guessing your looking at the Ford V10 or an older GM: I would estimate anticipated MPG at 8 to 10mpg. Lots of variables to consider. Driving over the Western mountains you will be lucky to get close to 8mpg. Your driving a shoebox, so a good wind will keep your numbers down. If you can hold your speed to the rig's sweet spot, perhaps 55 to 60mph all the better. Too many try to keep up with the cars, and 70, 75, 80mph will just eat up your fuel. A light weight 24ft motorhome will/should get better mileage than a 28 to 30fter fully loaded for a trip. Add a tow car? Since you want a large water and waste tank, consider that water weighs 8.35lbs per gallon. The more weight the rig is carrying the less mpg you can expect.
Yes, you want a generator. The slide really makes the RV more comfortable. Personally, levelers are a must, you will probably find HWH hydraulics on most coaches. I don't think you will find electric levelers on 28ft RVs or larger.
Water and waste: There are many places to properly dump waste, RV fuel stops, State rest areas, central dump stations at campgrounds, and many private campgrounds will permit you to use the dump for a fee.
If you really plan to stay off the grid for an extended trip you can also purchase a rolling blue waste tank, and transfer your waste to a portable tank and roll it to a toilet facility at a park.
Happy Trails.