I agree with the lower miles per-day posters.
My suggestion is to look for places along a route or nearby that tie into your interests, and pause a night or two, or even just a few hours. It really refreshes you during a long travel stretch cross-country. But it also makes the traveling memorable, not just a tedious means to reach a single place.
You could save ideas clipped out or jotted on paper in a folder for things to see en route for big trips you'd like to make - comments and info from this forum, websites, or from magazines about good campgrounds, scenic places, new attractions or historic ones. Factory tours, flea markets, specialty food/farms/winery regions, whatever you enjoy.
I've become much more interested in history since we bought the RV. There are free National Park Service folders with maps and points of interest for the Oregon Trail, Mormon Pioneer Trail, Pony Express, Erie Canalway, etc.
So far, we've followed the eastern and Midwestern part of the Lewis & Clark Trail. It was fun to camp by the water along the route and see a replica keelboat at Lewis & Clark State Park in Onawa, Iowa, for example. We hope to see the far western sites next year. We've enjoyed following Route 66, too.