I have 20,000 mi. on the OEM 05 Missions and they aren't even half worn. If you have a gauge, there is some brand variation, but new should be at least 10/32" of tread depth.
I have slight checking on one side. They've been covered since I bought, but I suspect this was the sunny side on the dealer's lot. They're also 4 years old now so I'll decide in spring when to replace them. The risk and hassle of a tire failure on the road certainly warrants replacing earlier rather than later.
I'd guess yours have a lot of life left--check the DOT code--it may be on the inside. The last 4 digits are the week and year of manufacture. Generally vehicle manufacturers do not get stale dated tires so the odds are yours are only weeks older than the coach and should be good for 4-5 years if not abused.
ST tires are not particularly susceptible to flat spotting. This winter, for the first time, I inflated mine to 60 psi since that is close to what they are running at on a hot sunny day anyway. Low psi is not good for any tire--sitting or driving.
2019 F150 3.5L Max Tow
2015 Silverado 1500 NHT
2014 Arctic Fox 22G
2005 Sunline T-2499