Adding 2nd battery question

[jjbbrewer]

What happens if I wire the 109 and 101 in parallel and use them?

The only thing you'll get is (109+101)*.8 = 168 ah of capacity. (DC batteries should only be drained to 20% of their capacity to prevent damage)

Seriously, forget about the diodes. They would be for preventing one very high capacity battery from draining a very much lower capacity one when connected together. The 24DC and 27DC are so close to being the same, there's really no concern.

The only "issue" is that after a few years of completely draining the batteries over and over, you'll have to replace the 24DC first.

(If it's any reassurance, I had this same 24/27 combination in my older toy hauler - kind of for the same reason as you. I needed another battery in Greybull, WY and all they had at the NAPA were 24DC)