The converter's output voltage will appear to sag under 12v loads so the idea is to keep it up above 13v (as seen at the battery down however long a wire that is) so the battery does not have to help out running the loads.
You could say the total 12v load of all the stuff in the rig on at once needs to be handled by the DC rating of the converter so it stays above 13v. Of course you don't often have everything on at once. Furnace on while running out the slide and raising the front jacks with all the lights and fans on?
That is why they have 13.6v as normal. When at 13.2 and you use 12v stuff, the voltage at the battery can go below 13 easily. Same on solar if you are on 13.2v Float in the afternoon with full batts and you run small 12v things, the battery charging stops and a draw starts, but if you raise the Float to 13.6, the solar can run the small loads and still keep some amperage to the battery so it stays up on the Float.
Some three stage converters will automatically trigger off 13.2 and go back to 13.6 when you run a 12v load in the rig.
