The compartments in my trailer are for Group 27 batteries and not tall enough for T-105s. You'll just have to measure yours.
Yes, there will be some rewiring because the 12V
batteries are in parallel and the 6V batteries must be in series.
Wiring batteries in series doubles the voltage, but not the current so the amp-hour capacity stays the same. Wiring batteries in parallel does NOT double the voltage, but DOES double the current so the amp-hours doubles.
Battery output drops significantly with cold temperatures, so if you can keep them in the internal compartments where they are warmer, you're better off. This is even more important if you are running the furnace rather than a cat heater in cold weather. Not everyone has internal compartments, though, and batteries on the tongue is the only choice.
If you have to put the batteries out on the tongue, at least the T-105s have a smaller base, which is important with limited space on the tongue. They make up for that in height.
Two Trojan 12V 27TMH
at 115AH over 20 hours, in parallel (230AH) give slightly more AH than two Trojan 6V T-105
at 225AH over 20 hours in series. Two warmer 27THM will give even more AH than two cold T-105s.
Hope this helps,