I wanted to let everyone know what I eventually decided to do in case someone Google's this topic and arrives at this post.
I spoke with multiple battery charger manufactures and electrical supply companies to find out that there are only really a few businesses that make DC to DC chargers that would work in my scenario. The 3 I mentioned in my 1st post seem to be the most popular, although there are probably more if you dig deeper into the Marine and Boating industries (they are used to charge "house" batteries from separately wired "starter" batteries).
By and large the "right" way to do this seems to be to just run a 12v
wire from the Tow Vehicle to the AS battery bank DIRECTLY (without any sort of in-line charger) that is sufficient gauge (roughly 10AWG in my case) so you don't have significant voltage drop. Running the charge wire directly from the TV to the AS puts the onus on the vehicle's alternator to detect the load on the entire 12v
system (which now includes the AS) and supply charging voltage accordingly.
In my case, I was worried about the alternator over-charging the AS batteries (a problem I thought a 4 stage DC to DC charger would rectify) however from my understanding over-charging would only happen if the alternator was defective (bad voltage regulator).
Another issue I was worried about was that the AS batteries would never fully charge from the TV due to the voltage drop from the TV to the AS. This is a problem rectified by using the proper gauge wire: thicker wire = less voltage drop = ability to fully charge AS batteries from TV. In my case, I want to use the existing 12v wiring on the pigtail from the TV, which is probably 10 AWG, and then use 10 AWG wiring in the camper to charge the battery bank. Depending on how many AMPs the TV's alternator is producing I can expect some voltage drop (maybe 3-6%) but hopefully not enough that it wouldn't charge the AS batteries; still not sure if they'll ever become fully charged in my scenario...
Airstream nor any of the other fifth-wheel or camper manufactures I researched use any sort of DC to DC charger in-line between the AS and the TV, which leads me to think that some engineer somewhere already figured this out and went the K.I.S.S. route on this one.
Thanks to all those that helped me arrive at my solution: no DC to DC charger needed, just run thicker wires and let the TV alternator do it's job.