That will probably go up as it heats up. You're getting a lot of passive cooling over the back side of the fridge. Ours is nearly empty, but when it's averaging around freezing, it tends to run for no more than 1.5 hours/day. I just checked, and it didn't come on today at all.
Most RV converters are 85-90% efficient. They tend to be notably less efficient at very low power, but then it doesn't matter as much then.
I think it's worth keeping an AGM in the system constantly floated at 13.8V. Pretty sure you can get DC-DC that will go 48 to 13.8V. That way you can handle the loads, and the battery will likely last a decade.
Most RV converters are 85-90% efficient. They tend to be notably less efficient at very low power, but then it doesn't matter as much then.
I think it's worth keeping an AGM in the system constantly floated at 13.8V. Pretty sure you can get DC-DC that will go 48 to 13.8V. That way you can handle the loads, and the battery will likely last a decade.