My 2015 3100 rl is listed at 13'5" and when I measured it hooked up it was 13' 4" and my rig appears to tow pretty level. I'm sure when the mfg. measures a unit it is level. If you have larger tires, a lift kit, or adjustable suspension you could easily increase the front height an inch or more, consequently if towing with a low rider
, highly unlikely, you could increase the rear height. Everyone should get an accurate measurement of their trailer while hooked up to their tow vechicle before they hit the road. An easy way to do this is to park under an awning that allows you to get on top of your trailer and still easily reach the bottom of the awning. Measure from the bottom of the awning to the trailers highest point, usually the front AC and then from the bottom of the awning to the ground, now just do the math
this method eliminates the risk of not having your board parallel to the ground when using the 2X4 on top of the AC method. Unless your truck sits higher than stock, you're pretty much safe to go under anything a semi will, as unless built for special services they are all 13'6" or less.