No to a power washer. Use any soft mitt or brush on a stick you would use on your cars. Sounds like the exterior is in great shape so any spray on wipe off detailing product will keep it looking new for a long time. The easier it is to spray and wipe the more likely you will do it. Find one that has some uv protection such as aerospace 303. Wash windows the same way you do your cars.
2016 Newmar Bay Star Sport 3004