If it's been cared for reasonably a rubber roof should not be a concern. I had a 2005 Milner for 10 years and my rubber roof was perfectly functional when I sold it. I used 303 on it regularly and it seemed to preserve the roof nicely.
Hallmark does replace rubber roofs and includes new soft walls also I believe but it is expensive. Hallmark told me once that most of the replacement jobs are related to insurance claims because of the expense.
Either way, should you purchase the unit I hope you have nothing but positive experiences with it. Hallmark makes a great camper.