Tire tread depth when new

The age by DOT date codes stamped on tires and depth of sidewall cracks are the usual ways of determining whether RV tires need to be replaced. Tires over 5 years old and with sidewall cracks deeper than 1/16 inch are considered unsafe.
If a tire comes apart underway, the flailing steel belt can sever propane lines, crack holding tanks and wipe out wiring as well as causing loss of control of RV. Many RV's don't get driven more than the average 5000 miles per year but tires suffer damage from sun and weather when parked for long periods.
Rental RV's probably get driven more miles per year than privately owned ones so tires on rentals may be OK even if more than 5 years old. Driving lubricates the interior materials of RV tires. Above is what we are told by tire companies.

Thank you all for the information

The standard, at manufacturers and dealers, for 50% wear is not usually 1/2 of original depth, rather 1/2 way between original depth and minimum depth as indicated by the wear bars, usually 2/32 inch. Of course, these guys want to sell you new tires as soon as possible.

Legal minimum depths can be as high as 4/32 for commercial heavy truck steer tires, or as low as 1/32 (e.g. California inspection standard for private passenger vehicles).

Original tread depths might be as shallow as 6/32 for performance tires for passenger cars (possibly even less for tires with very high speed ratings) to more than 13/32 for commercial truck and trailer tires with lower speed ratings.

The Michelin MS/2 in Load Range E (an "LT" series tire) has 13.5 32nd's of an inch according to their webpage. So around 6/32" would be about 50% wear.

You can usually find tread depth on the tire manufacturer's website. It can vary by tire model and manufacturer.