When I bought my used RV, I considered the battery and tires to be standard items to replace in short order. Unless they were rotted out completely, I see it as an expected expense in addition to the purchase that I, as the buyer, will need to make. I don't expect the seller to have to buy new tires for me.
I would say have the tires checked but don't make a presumptive strike. Simp!y tell the buyer that as they are 7 years old, you had them inspected as recommended and here are the results of that inspection. Unless the inspection says they won't survive a drive to the corner, then just provide the results to the buyer.
The exception to that would be if the buyer came from a long distance and needs to drive it home. In that case, I would mention that it might be good to get new tires first and you'd be happy to refer him to your preferred tire shop and maybe take him out to lunch while its done.
Yes, RV tires are expensive. But they are a recurring expense and as long as you have been following good tire practices (i.e., not refraining from buying new tires for 2 years of your use because you figure you can eek out the last viable usage and sell it with bad tires), then it's the buyer's expense.