CruiseAmerica is the only natioal rental company I am aware of doing one-ways, and it take substantial advance reservation sometimes, and there is a one-way fee. The one-way fee isnot going to be as much as the mileage would be foe a two-way. Indeed, mileage will likely be your big cost, as daily allowances on RVs tend to be small,100-200 miles a day.
Six people, a 30 C will likely be more usable for six adults than any A much under 40 feet, as the two types are in different markets, with most A's designed as home for a couple, road trip for a family of four.
I am assuming you'll want to cook and sleep in it. Otherwise, a small travel coach or party coach might work better. I know of bus companies that have coaches with capacites from four to forty passengers, other up to about 8-12 with sleeping and lounge combinations. Since these are mostly used by entertainment road crews, many of the companies operate out of Nashville, but I know I can hire something like from Dallas, Oklahoma City, Peoria, Kansas City, considering the transportation companies I've traveled with. From KC, we've used Kincaid and Trailways. Starting from Atlanta, I would check for transportation companies operating from Atlanta, or may Jacksonville or Orlando.
These will not, howver, be rentals, they will be charters with drivers. They are priced accordingly. If you want a non-stop cross country run, you'll likely have to pay extra for two drivers, because of the mandatory rest periods.
Six people, long trip, that's usually executive transport category, something about the size of 18-30 passenger airport bus but with bar and lounge facilities for a smaller number of people, rather than full of seats. If you want bunks aor sleeping enroute, that is more or a crew coach, with bunks for four to eight people, lounge seating, entertainment type kitchen, toilet and maybe a shower, in a 38-45 foot motorcoach shell.
Just some alternatives to the RV/motorhome option, from someone who does it time to time.
