Here's something that should carry over to all of the buses with tires that are in great shape, but look old and worn.
Now the tires on my truck are old. . . . . They hold air fine, carry all the weight I can throw at them, been wearing evenly, great all around traction but, they are fading to grey and have dirt mud and salt stains that just don't seem to scrub out. Armor-all is a waste, lasts a day and it's gone. Those tire black treatments are not worth the money on these old tires. So here's what I did about 20,000 miles ago and been doing it since. Cost me about $15 for a brush and some cheap black paint from a commercial/industrial paint supplier. Yup, bought a gallon of paint and a brush and about twice between oil changes I brush on a thin coat of paint on the tires. Yeah, it's bootleg. . . .but I've used about a quart of paint so far on these tires and they always look armor-alled.
It seems like those heavy duty truck tires found on buses last almost forever in a motorhome setting, but the looks may not be there. Grey fading, dirt stains, small dry cracks. . .all things that make a perfectly capable tire look old and worn.
Lets hear some thoughts, opinions or call me just plain old cheap, but hey. . . .it works. Here's a picture.