The normal voltage drop would be a function of wire size, wire length and amp draw. The more amps you draw over any given wire size and length the more voltage drop you see.
The smaller wire for any given length and amp draw the more voltage drop you see.
The longer run of wire for any given wire gauge and amp draw the more voltage drop you see.
For instance a 30ft run of 10awg wire pulling 30amps would see a voltage drop of about 2 volts.
So if your batteries where pretty dead, or you had a bunch of DC powered stuff on in the coach when you took these measurements then I'd say you're well within spec.
In the gorgetown's I've seen the converter is at the left rear of the coach and the batteries are under the step, almost on the opposite corner of the coach. That's a pretty bad setup. It would have been much better for them to mount the converter right next to the batteries and run a 110v line up to power the converter. It would have been cheaper in labor, and wire cost and would have had less voltage drop so the batteries would have charged faster. I don't understand why they did it the way they did it.
Here is a link to a voltage drop calculator:
Voltage Drop Calculator