Replace RV tires at 6-7 years regardless of tread condition. Internal rubber degradation is not visible but increases blowout risk. At 7 years with 2018 DOT codes, your tires should be replaced before extended travel.
Replace RV tires at 7 years maximum regardless of tread. Check DOT date code for manufacture date. Rubber degrades over time increasing blowout risk even with good tread depth.
Upsizing to 275/70R22.5 from 255/70R22.5 is possible if clearance allows. Measure wheel width (needs 7.5+ inches), check steering clearance, and verify load rating before changing.