Tony: Those numbers do look correct.
The issue comes with the required clearance above and below the unit. 6 inches in each direction for ventilation purposes.
I do have an e-mail out to Renogy. Perhaps they will say that all this clearance is not necessary or I can install this horizontally, but that would be against their pdf recommendations.
If my tongue box is a dead end for installation I have noticed online that there is a large variety of tongue boxes with depths exceeding the assumed required 20 inches.