300 miles or so is about our limit per day. Driving on rough interstate concrete surfaces curvy mountain roads, hot weather can be extra wearing on driver and passengers.
Try to drive during the cooler part of the day in hot weather. Stop every 100 miles or so at a rest stop, etc. Look for an RV park or forest camp site around 3:00 PM and get settled before dusk. Call ahead to reserve to be sure in peak season. ( Don't get caught dead tired and hungry driving around in the dark seeing "no vacancy" signs.) Not all Walmarts allow overnight parking. Some police stations allow it. Overnight parking at rest stops is not always permitted and may not be safe.