I have been having this same conversation with our dealer and our Jayco rep in regards to our "yet to be delivered" Eagle TT, especially in units that are sold with a residential refrigerator.
When I inquired with our dealership, on how many and what type of batteries would be in the trailer at delivery, I was told that it was the same one they sold with all their TT's, which was one group 24 Interstate deep cycle. I then asked if that was the case with the larger 5th wheels like a North Point, Pinnacle, or Designer and he said, Oh no, those have 4.
Now wouldn't you think that smaller TT's/5er's that due to their installed options, or much higher rate of being used in a boondock situation, be the ones that actually get the batteries needed to operate in such an environment? Or at the very least, be asked by the salesman, what the expected use would be? Guess not.
And even during my discussion with my Jayco rep, he stated that they test/certify the residential refrigerators using 2 deep cycle batteries, but was also surprised that I was only going to get one. He also didn't state whether this type of information is passed to the dealers or not.
So as far as I can determine, it is up to the salesman/dealership to make the call on how many batteries are sold with each unit, however what that is based on looks like it has more to do with the price point of said unit and less to do with what actual options are chosen, or how it might be used.
2015 GMC Sierra 2500HD SLT 6.0L SRW
2017 Jayco Eagle 330RSTS (on the way)