19.5 Truck tires

I would go by age since you dont seem to do many miles per year.

Not sure about your tires but generally manufacturers suggest replacement after 5 years. The issue is UV degradation of the rubber, not wear and tear.

Given how you care for them and the fact you dont live in an area that is especially hard on tires (UV), I'd run em until they start cracking or checking.

5 years? Lol.
Maybe after 10 I'd think about tossing them.

Inspect, inspect, inspect.
Sun and weather-beaten tires will show cracks on sidewalls.
That still need to be evaluated as hair cracks on car tire are signal to replace, while commercial tires have 1/2 or even 3/4" of rubber on the side and even deep cracks don't make them bad.
Rule of thumb is 6 years for car tires, or 10 years for HD tires, but I drove just fine on 10 years old tires on car that was always garage -kept.
19.5 are commercial tires and kept in the garage, so I would give them 10 years easy.

That is what my tire guy said when I got my 19.5's. He said they should last 10 years.

Give them a thorough inspection yourself, easy now since they are off the vehicle and stored inside. If you find any concerns, take them to a reputable tire shop (not a Canadian Tire Store) and have them inspected.
Up here the UV rays are much worse and much longer exposure in the summer than in the winter, personally I would never think of bringing the tires in for the winter.

Ken

Here's a link, not completely authoritative by any means, but ten years seems like a reasonable expectation given the way you use your tires.

https://www.discounttire.com/learn/tire-aging

I would not use tires past six years of service, period. Saw a motorhome on the side of the highway yesterday with a blown out steer tire. That is any driver's worst nightmare.

Agree. I was always told when I was running a 26' class C to change the tires every 5 years no matter what. Took me having blowouts with the family headed up 395 twice from trying to stretch to 6 years to learn. Tires had less than 8K both times, looked great from the outside. I learned to listen to experience finally.

Not fair comparison.
26' motorhome build on suspension design to hold 22' chassis. Those chassis have LD tires from factory and RV builders don't upgrade the tires when they add 30% more length.
Some motorhomes at least used to be build having 300 lb cargo capacity and they stay under the load 365 days a year.