Batteries charge at different rates, depending on their initial state of charge and temperature. With a modern 3 stage converter/charger (and I have no idea if your trailer has one or not) in general the charge starts with a Bulk charge, stuffing as much into the batteries as they will take and brings them up to probably 80% full when done. Then the converter/charger switches to an Absorption charge which is at a much lower rate and brings the batteries up to maybe 92 to 95% charged. Then the unit will switch to a Float charge, which does the final 8% or so. This takes many hours, often 24 or more.
If you don't have a modern 3 stage charger in your trailer, it will take far longer to charge the batteries, as a simple charger will not put much into your batteries at all after the first half hour or so.
So, what is "fully charged"? To get to truly full charge might take a day or more of generator operation. To get close enough, 80 to 90% charged, takes far less time. Your generator may (not sure) go into eco mode when the bulk charge is done, or about 80% charged. I say I am not sure, as the only real way to monitor it is with a good voltmeter, attached directly to the batteries (so there is no voltage loss as you would get with a plug in voltmeter). Also, as stated above, you may not have a 3 stage converter/charger. The other problem is that the batteries need to rest, not being charged or discharged for about an hour prior to measuring the voltage in order to have any meaningful results. That is not really practical in most cases. Battery state of charge tables vs. voltage are commonly available, but are often misused as the temperature is different from that of the table base.
A Progressive Dynamics converter/charger with charge wizard has a blinking LED light which will tell you which stage the charger is in, Bulk, Absorption, or Float. Other converter/chargers may not have that feature.
Bottom line guess: Your generator probably needs to run 2 to 3 hours to bring your batteries up to 80% charge, with a 3 stage converter/charger. Many more hours to get to 92% and a day to get them truly at "full charge". With a simple one stage converter/charger double or triple the times mentioned.
That sounds like an evasive reply, but there is no simple answer to your question.