How far out of your way are you willing to go? Lake Mead is sort of on the way, Glen Canyon NRA is somewhat out of the way, Curecanti NRA is even further off the most direct routes, but is one of those special water and forest places that are rare in the west.
Waterfront campsites? Except for a small number of sites at some of the CoE flood control reservoirs, parking right on the water is not very common. Those goRVing photos of campers parked alone on the grass at the edge of a little lake are part of the myth of RVing, at least my experience of it, and finding a spot for a 40-foot bus that is towing will be more difficult.
Work out a route that takes you across US-50 through Colorado, and you will find some beautiful places in high desert, high altitude scrub forest, and at Curecanti, on the largest reservoir in the Rockies, but no guarantees that your campsite will be at water's edge, as those, where they exist, are usually first come, first served.
If you want to come about 2000 miles out of your way I can find you waterfront sites on the Arkansas River flood control system in NE Oklahoma, SW Missouri, NW Arkansas, but you have to be there waiting to pull in the moment the previous guy leaves.
Tom Test
Itasca Spirit 29B
