Re: AK/WA USDOT requirement?
Do not waste energy worrying about this.
If the gross weight is 26,000 or less, and the bus seats less than 15 passengers, it is not a "commercial motor vehicle" according to the uniform standards set by the Feds. If you are registered as a "bus," be sure to pull out any seats for 15 or more passengers, so you will not need a CDL. The feds also declare that "Recreational Vehicles" are not commercial motor vehicles.
If you are still worried, putting "Not for Hire" or "Private Coach" on the sides could be used as added insurance, if your bus still appears like a working commercial vehicle and not a private vehicle. Once you have painted it other than yellow, installed RV air conditioners on the roof, skinned some windows, added happy graphics, or made other obvious custom changes, I would not worry at all.
In any event, if it is not used commercially, do not pull off the road in to scales or safety inspection areas. Keep right on going unless waved over by a police officer. If waved over, show your non-commercial documentation, but do not argue. You should have safe braking, tires, wipers, horn, lighting, etc. regardless of use, and law enforcement has the right to check any vehicle on the road, regardless of type, if there is "reasonable cause to believe" that it has not been maintained properly.
Someone said "Making good decisions comes from experience, experience comes from bad decisions." I say there are three kinds of people: those who learn from their mistakes, those who learn from the mistakes of others, and those who never learn.