I would consider a low mileage unit to have between 2,000 - 3,000 miles a year. So for a 2004 you are looking at 30,000 miles.
Be careful. Mileage is not always a good indicator though. If an RV has sat for a long period of time unused there could be more major mechanical problems than with a higher mileage unit that has been used regularly.
Tires may last five or six years. Most RV tires rot before they are worn out. If the units tires are five years old or older figure in a new set of tires. If they are younger, look for cracks in the sidewall.
If they are Michelin tires made between 2010 and 2012 look for this
code: B3JH AKEX (0210 to 2512) They will replace them at no cost and you will have a brand new set of tires.