I would revisit the two 6v GC battery application. You could keep your existing 12 v for backup if you're worried about 6v failure.
The mixing of two different batteries in parallel may be a charging issue. Your Grp 27 doesn't have 160 AH measured at the conventional 20hr. rate, BTW. Mfg's are using a different measure and pasting that on the label so that it appears that the buyer is getting more for his buck, it isn't so. I would guess that you have 70-90 AH at the 20 hr rate.
I have heard of one Forum member having a failure of 6v batteries. There have probably been more but my experience with 6v batteries tells me that they are the most durable SLA battery.
If you opt for the 12v batteries, I would buy two new ones and, as you mentioned, not at the RV service place.