Group 31 is the correct size for the engine cranking batteries if they are at least 950-975 CCA each.
House batteries should be at least group 27 size, and if you can fit group 31
DEEP CYCLE batteries (not the cranking specific style of the engine batteries...) that would be even better.
The group 27 batteries your friendly dealer put in are probably going to work OK as "house batteries". For contrast, the 6 volt batteries are at least 205 AH per 12 volt stack and the XDH you had were 232 AH when new. Because these are used in series to make 12 volts and the front of your coach allows two parallel banks of these two 6v stacks, your inverter would have 464 AH at its disposal. Using 12 v group 27 batteries all in parallel gives 4x approximately 90 AH or only 360 AH total -- quite a large deficit over the designed configuration...
Clearly you can also use these golf cart batteries for house batteries by stacking two for a 12 volt 232 AH bank. This is likely more than you can obtain with two 12 volt batteries of any group that would fit, but you need to modify the cabling and hold down brackets.
Family Motor Coaching is a good source for information and you may wish to join their organization and subscribe to their publication. Here is a battery specific article from some years ago that still has useful facts:
https://www.fmcmagazine.com/images/s...ce_jan2012.pdf
Chuck