Contact the manufacturer of the tires...
OK, the best person to report on the condition of the tires as related to their age is the manufacturer of the tire, period.
So work with the dealer to get the DOT date codes and serial numbers from the tires installed on the unit as well as the weight statements from the vehicle tag, photos of the tires (tread and sidewalls) and send off an email to the manufacturer of the tires.
Take a tread depth measurement as well.
List the exact model of the equipment and include the axle weights and include the photos and other data.
Copy the dealer as well so all is on the up and up.
Ask the manufacturer to specify the usability and safety of the installed tires and have them define weather or not it is safe to use the MH at highway speeds.
You could ask the manufacturer to estimate the percentage of tire wear based on the tread depth, so if there is 50% of the tire left then you could factor this in with the dealer if you wish to allow it.
Also ask them to identify a suitable current replacement from their line as well as a local source with the replacement cost (installed on the MH)
Given the expectation for tires to be replaced based on age or tread, and the fear of a lawsuit the manufacturer will most likely state in very clear language that their tires are of the very best quality but due to the age alone should not be considered for use on the road and should be replaced.
Both you and the dealer now have a clear document of what the tires are worth and can go from there.
But given the current economy the dealer should be willing to get creative to move the sale, you may not wish to allow them to discount the wear on the tires, but what is the real value of the MH?
Is the price asked in alignment with the condition with the current tires and everything else? is it too expensive, too cheap or just right?
If the dealer is being dificult here there may be other concerns with the dealer.
Bigger concern is WARRANTY!
If tires are 10 years old then what about the other systems in the MH?
Working with the dealer on extended warranty would be a good lever to pull as well.
Remenber, YOU have the money and he wants it very badly, you are in 100% control of the deal, you can walk away with your cash, or with a bad deal, your choice!