I see new campers heading south to dealerships all the time. One at a time, being pulled by some contracted driver. Very seldom do I see them loaded on a flat bed. And then its the smaller a frames or single axle TT's.
Only small units are transported on a flatbed, mostly single axle units and the lightest tandem axle units. Anything over probably 25 ft is towed. So, your unit will have mileage equal to how far you are away from the factory. Drivers are not allowed inside the unit, so the inside will be determined by how it leaves the factory. I doubt drivers even check tire pressures, just hook it up and go.
They are transported one at a time using contractors. That's one of many reason's why it's a good idea to change the china bombs as soon as you get your trailer. depending where it was manufactured those may have substantial mileage on them. Pop ups and a few smaller trailers are transported by truck.
Retired Fire Dept Battalion Chief
2016 Ever-Lite 232RBS
2012 F150 FX4 W/Max tow & Ecoboost
It depends a lot on the size of the trailer. Yesterday I watched three of the new retro Shasta trailers going out on a flatbed. I've seen as many as eight or ten popups or hybrids going out on a double deck trailer similar to a car hauler. The larger trailers and fifth wheels, of course, are pulled individually.