I also would use the existing tire for the drive home provided it looked safe. Most tire companies suggest that you have tires professionally inspected annually after five to seven years, and only replaced outright after ten years or if the inspection shows problems. Many (but not necessarily all) six year old tires are perfectly safe to use. Do check the pressure in all the tires (preferably including the spare) and correct as necessary before heading off.
I'd also probably leave the off-brand tires alone, at least until I saw how they performed and rode. Spending $700 or whatever to replace perfectly functional tires seems silly to me.
By the way, a great many class C owners do not rotate their tires ever, since there's typically no real need to exact the maximum possible tread life out of the tire (they usually time out long before the tread is gone) and duallies tend to complicate the tire rotation schedule for various reasons. It's best for the paired tires in the back to have the same sort of wear, since having one be a slightly different diameter than its partner will inevitably lead to some amount of scrubbing while driving. Also, if there are any special valve stems or similar, they're often tied to a tire location.
