I would price the coach at whatever you think it's worth and the market will bear. I would get a estimate on what 6 new reasonably priced tires would cost and be willing to adjust the sale price down to include tire replacement.
I would be very fair and honest about what the coach needs, but I wouldn't advertise that it needs tires. If asked, I would reply, yes it's ready for a set. You just may find a guy who nitpicks every detail on the coach, is a complete jerk and wants a discount for every mark or dent, yet misses the tires. Play the sale by ear, don't lie and adjust the price to what works for you.