I think the advice to consult a tire shop for an expert diagnosis is sound... in theory. Sure that sounds like a great idea.... but I have pretty close to zero confidence that most of the folks in tire shops will know (or care). My bet is that they will all point to balance as the primary variable and as a baseline will suggest spin balancing right then in there....and if/when that fails they'll suggest the tires are bad and need to be replaced.
I've had cars before that after sitting for a while, especially in the cold, would take quite a few miles to smooth out but I don't have a mental note of how long it took. After 10 miles I would think that they would be at least a lot better... and if yours were no different then yeah, maybe there's a problem....
Were the tires aired up well the whole time it say or was there ever a time when they sat flat or nearly so? If that was the case then I would suspect damaged tires and would probably just replace them right away. These long class C's are HEAVY. My drive axle tires sit basically at or over their weight capacity all the time, so I don't think it's wise to play with bad tires. Blowouts can cause lots of damage (I have first-hand experience)
Are the tires really just two years old (by the date code)?
If so, I think that what I would probably do is drive it a bit more after giving them a really good look-over. Crawl under and have a look at the rims, between the dually's, etc... Make sure there's no cracking or anything like that, no dirt or rocks or rats nests caught between the wheels, etc... Double check the pressure, then go for another drive, preferably on a warm day..... You said it was better under 20mph, so I'd try to find a place to drive 25-30 for a while, then gradually speed up...35, 45. Stop-and-go-in-town type driving might be a good way to work the tires while keeping the speeds low. I think that I would go more on time than distance at those speeds but I have no idea how long (or far)...just to make something up maybe 30 minutes to an hour.
If it seems like it's getting better then I'd probably keep trying longer but if it was still the same after that then I'd be visiting a shop to get the tires pulled, inspected, reseated, spin balanced, and the alignment checked.
If you do replace them, on a 31 class C, I'd strongly encourage you to get C-rated commercial tires instead of the LT tires that would have come on the thing originally.
these Kumho's are the tires I put on mine a little more than two years ago and I've been happy with these. Sleep a little better knowing they have a higher load capacity and should be a bit more durable...I don't cringe quite as badly if I hit a rough spot in the road....
good luck!