Mileage on tires will vary a lot, depending on the specific tire model (thus its relative expected "tread life"), how it's driven (hot rod, Sunday-go-to-meeting), frequency of operation and maybe other conditions, but cars are generally in need of replacement with 2/32" of tread depth remaining, big trucks with 4/32" and assuming otherwise in good condition and haven't aged out (see below).
Typically RV trailer tires (5th wheel, Travel Trailer) should be replaced around the 5 year mark, and most motorhome tires around 7 years, given that RV tires rot from the inside because they don't usually see enough miles to wear the tread down, so they tend to age out.
So if Mike thinks they need replacement, chances are good that he's right. Safety and vehicle integrity suggest that you err on the side of caution, given the possible consequences.