My 5,000# truck and 10,000# capacity trailer combination certainly doesn't require a DOT number...and if it is USDOT, that isn't just a Kentucky thang. Heck, nearly any 3/4 ton or 1 ton truck towing any type of trailer would have a rated load of over 10k lbs. MY rig exceeds 10k lbs with either a moderately loaded utility trailer or my travel trailer in tow.
Don't see many DOT numbers posted on vehicles in campgrounds...by this calculation we'd all be illegal. We can't ALL be illegal...
Is this just commercial use vehicles?
From Maine DOT website;
"A pickup truck registered as a passenger vehicle may not exceed 6,000 pounds gross vehicle weight unless it is towing: 1) a camper trailer; or, 2) a utility trailer weighing less than 2,000 pounds gross weight where the combined gross vehicle weight of the pickup and the utility trailer does not exceed 8,000 pounds. Otherwise, pickup trucks towing trailers where the combined gross vehicle weight exceeds 6,000 pounds must register as a commercial vehicle."
And
"Qualified Vehicles. Vehicle operators using fuel other than gasoline must establish an account for fuel tax licensing and reporting if the motor vehicle:
1) Has a registered gross weight or actual weight of more than 26,000 pounds; or
(2) Has three or more axles on the power unit regardless of weight; or
(3) Is a bus with a seating capacity of 20 or more including the driver.
Each qualified motor vehicle must display either a Maine fuel use identification decal or an IFTA decal.
IFTA. (
IFTA, Inc.) Maine is a member of the International Fuel Tax Agreement (IFTA). Anyone operating qualified motor vehicles in two or more IFTA jurisdictions must license and report mileage and fuel consumption in all IFTA jurisdictions through their IFTA base (home) jurisdiction. A Maine-based carrier operating out-of-state must license for IFTA with the Bureau of Motor Vehicles Fuel Use Licensing Unit unless the carrier can satisfy the basing requirements in another jurisdiction. In IFTA, the carrier is issued one set of decals good for travel in all IFTA jurisdictions, and makes one quarterly fuel tax report to the base jurisdiction. Many IFTA jurisdictions require qualified gasoline-powered vehicles to license and report. Maine-based carriers with qualified gasoline-powered vehicles will license and report through the Bureau of Motor Vehicles Fuel Use Licensing Unit.
Intrastate Carriers."
RV's do have certain exemptions; a quick search didn't produce them. But ask any trucker, he'll be sure to point them out.

(I've sure heard them enough)