We usually travel no more than 250 miles a day at 55 MPH. What series tires do you have? E-series tires are rated at no faster than 65 MPH. We are fulltimers and are rarely in a hurry to get anywhere and fuel mileage is better at 55 MPH.
We have routinely stayed overnight in Walmart lots and always call ahead to make sure the local municipality does not have any restrictions. Yes, the lots can be noisy, but it has not been a big issue. We stay hooked up and lower the front jacks to increase stability. Because we are only there overnight, we don't run our generator and only use the 12 volt system.
It has not been our experience that you can routinely find a reasonable expensive campground that will guarantee a pull-through site.