Stated wheelbase

When you pay a toll they measure from the center hub on the steer and the center hub on the tag. If you want another way is from the center hub front to the center between the rear axle and the tag.
Or center hub rear to center hub tag, the free space between the rear tires.

I just use the the front to the tag. A lot easier.

MountainAir5, thanks for the reply. So basically you're not aware of any standard that mfgs. adhere to when reporting the wheelbase for any of the tag axle rigs they produce.

Look at this one. I have seen older unit that were over weight before you purchase them. Some of the newer ones that puts in everything a person thinks they must have also go over. just look at the weight of the rV and go to the state you want to travel their roads and you might get shocked. With a tag your max will be close to 45 and even maybe a 43. Our last RV had tag and was 43. Just look at the spec for length for the brand you are looking at.

https://oversize.io/regulations/axle-weight-calculator

My original post may have been misleading. I'm looking at a rig on the internet, aside from many photos there also is a spec sheet that identifies the wheelbase is 288".
Is the 288" the distance from the front wheel hub to the rear wheel hub, or, is it the distance from the front wheel hub to the tag axle hub?

I think you will have to measure the unit. Just looking at say Newmar Mountainaire, you can get a 40 and 45 foot with tag. Depends on what you order. I can not tell you what you would get for the 288 inch wheel base. Just measure from front axle to the rear or tag if it has one. I would say the tag. Probable not the answer you are looking for.

Better stated than what I stated.

https://en.wikipedia.org/wiki/Wheelbase

Just measure what you have when its park on your slab and see where the 288 lands.

MountainAir05, thanks for info.

This is how Freightliner does it....other's do it to the center of the dually wheels and the tag and others to it to the center of the tag:

On trucks with multiple rear axles, wheelbase is generally measured from the center of the front hub to the center of the hub of the most rearward drive axle.

Wheel base is center front wheel to enter drive axle on my chassis