Thanks for the suggestions, I greatly appreciate the help!
This model didn't have a tub, so I spent a few hours digging around under the back closets, and de-assembling and re-assembling them. On the downside, the battery wasn't there, but on the plus side, I did eventually find it.
Although it makes me look a bit silly, I figure I should share where I found it, in case someone else searches this page for it years from now 🙂 The battery was in a really convenient location - It's right under the bed/couch. It was in a Vented RV Battery box, and I somehow convinced myself that was part of the furnace system, not electrical.
I really appreciate the help and advice. I had nearly convinced myself that somehow THIS Horizon didn't have a battery, but I'm glad I kept digging.
Thanks again.
CPD