Figure 10' for planning purposes. If it's close you should measure the actual trailer.
The clearance on my 2010 classic was supposedly 9' 7.5" when new, and that's with low-profile A/C. Most trailers are very close to this. Tires, axles, and differences in air conditioner height can each lead to changes of a few inches. Low profile A/C is 9.5" high. Standard is 12.75". You can lose half an inch if there's a double gasket.
It's difficult to measure clearance accurately, and even if you do, suspension travel, hitch height, and uneven ground can cause you to hit stuff that the clearance measurement says you would miss. Be careful.
To learn to see below the surface, you must adjust your altitude