X2! I've done this twice at the truck stop CAT scale in Tucson. DO park and go in to explain what you want to do, before you drive on to the scales.
In my case, the second time I did it, I asked for two weighings, one with the truck and trailer connected, and the second with the trailer disconnected.
There are three scale platforms. You'll be able to put your tow vehicle axles over the front two, to get an accurate measure of the load on each, and the trailer on the third.
The cost is minimal. I recall it being less than $15 per weighing, but that was some time ago, so my memory may not be accurate.
- Jack