Look for sidewall cracks - if the sidewalss aren't cracked and the tread area is still thick - you might be able to get a couple more years out of them.
$3000 sounds WAY HIGH. My last set of seven (including the spare) Michelin M&S2 tires for our Class C cost only a little over $1800.
By the way, both the Michelin M&S and M&S2 tire lines have now been replaced with their M&S "Defender" tire line ... so if you do get a net set of Michelins be sure and ask for their latest M&S Defender tires. I have a new set of these on our rig as of early this summer.
I use the 215/85R16 Load Range E Michelin tire size on our Class C so as to get more ground clearance and less rolling friction, while at the same time having the same weight carrying capacity.