To avoid the amps tapering issue, I have run a smart charger from an inverter on the truck battery.
Firstly, the inverter has to be rated enough watts to run the smart charger. Secondly, the truck has to be able to run that inverter with that load.
Given all that, I found our truck can run an inverter that is running a 35 amp Vector smart charger, that in turn is charging at 35 amps (constant amps no tapering) with the truck idling and parked for an hour or longer.
You know right away if you load the inverter more than the truck can supply it with DC when the truck's voltage drops off from 13.9 and dies off with the extra load. (Also turn off the truck's 'auto climate')
The whole thing being that the Vector charger does not taper its amps, so it is way superior to the 7-pin tapering)
All you need now is to have the inverter off the truck battery supplied by its alternator, powering a smart charger, charging the trailer battery going down the road, with your fridge on the trailer's own inverter powered by the trailer battery being powered by the Vector charger from the truck. :)