Welcome to the forum. Congratulations on joining the Breezeway. Info here, on Facebook and on the Tiffin Forum.
Here is one of the several IRV2 threads on House batteries. Use the search box at the upper right to get more options and recommendations.
Since you are new to the coach, I would replace with 2 golf cart 6 volt deep cycle cells from Walmart. If they fail, you can get them replaced at the next Walmart you see. After you have more information and experience with the rig, you can consider a wider range of battery alternatives. Actually, just about any 6 volt deep discharge cells that you can justify purchasing will do the job.
If you can upgrade to a bank of four cells, you may be happier with the capacity.