You should have an engine starting battery(s) and house batteries. I've never seen a class A with only one battery that performs both duties.
You changed out one battery - I'll assume it was a 12V? Both the house and starting system works off of 12V. The engine can use one or two 12V batteries. On the house side, while in a small trailer, some of them get by with one 12V battery but on all other larger rigs, it's two or four 6V batteries wired in series and parallel.
In the battery compartment where you changed out the one battery, you said there were no additional cables, but was there room for additional batteries?
Bill