A few things come to mind. If this were a boat or a motorhome, the batteries would be close together, and there is an advantage to having the batteries combined (like one big battery) during charging, as the alternator will see the combined voltage and charge at the rate needed to charge all batteries. The charge will favor the battery that is the least charged. When the batteries are not being charged, the combiner separates the batteries, then the house and starting batteries are separated so there is always one in reserve for starting. Not so-or needed on a TT as the TV is disconnected while camping.
On a TT the battery is 25+ feet from the alternator (really 50+ feet as you need to run a ground wire back to the battery, or a ground wire from the TT battery to truck frame and one from the TV battery to the TV frame). There will be a lot more resistance in that loop of wire, even if it is 8 gauge, than the loop of wire from the TV battery to the starter and alternator (about 6 feet total). The big problem here is that this resistance difference will cause the voltage regulator on the TV alternator to favor seeing the voltage in the TV battery and charge accordingly. If the TV battery is totally charged, the alternator puts out very little and even less can get back to the TT battery. In my opinion this is the main cause for the TV not charging the TT battery, not wire size, but the smaller wire size in the TV charge wire makes this condition even worse. I see no big benefit to using a combiner to connect the two batteries, maybe a larger gauge charge
wire loop and a larger amp capacity relay. Much cheaper. If you really want to charge your TT batteries quickly from your TV you will need a dual output alternator and a increased capacity charging circuit. Thats how we do it on boats.