I'm not surprised with the 308RETS, they can vary from one model to the next. A number of things can impact coupler height: Tire size, type of axle, type of slide mechanism, height of slide, not loaded, A-frame width, etc..
I wonder if there is a standard coupler height (or dimensional window) that RV manufactures design to??
2016 GM 2500HD 6.0L/4.10
2005 Jayco Eagle 278FBS
Reese HP Dual Cam
Putnam Class V