Yes it is a buck converter, voltage drives current (can't flow current without voltage differential) so dropping voltage is the only way to limit current. It is a CC / CV unit, you set either limit. Agreed, it is not a constant current device, so it will not force the current by pushing the voltage up, but it will cap the maximum amount of current flowing through the circuit.
We have not seen the output having anywhere near a 5V drop. More like ~.2V
I do see the spec does limit input to 40 V, and output to 35 V, but I do not believe that is stating this version has a minimum of 5V drop to function.
I may not have linked the specific unit we have use as a CC limiter, but the one we have only drops .2V on 12V, the unit linked is 300W max, so will handle up to ~20 A at charging voltage (unless you set it to limit lower than that). But the unit linked does have CC output, so you can set the current limit as desired.
The concern on a buck/boost is that if the vehicle battery sags for any reason, the unit will still continue to charge the trailer. Not the end of the world, unless you forget and it draws down the vehicle. So would work also from what I've seen.
And since charging system temperature compensation is relatively newer, I agree, setting at a constant 13.8 V is a simple solution.
All that said, I have not yet used one of these in this specific application, so YMMY, previous performance is no guarantee of future results, etc...