I am not sure I always believe what the readouts say on this stuff. Sometimes the Schwintek slide outs can be "recalibrated" by retracting them (or extending them).
The video below may or may not work, but it is a simple thing to try. For example, if the slide is out of alignment, it could be demanding more current from the motors (especially if it is binding at some point), which can reduce the battery voltage if the load is too great - and be reported as a low battery.
Also, if you have two coach batteries, you should always replace them both so they are the same age, with two identical batteries (chemistry, AH rating, etc). The reason being is they operate in parallel without any kind of isolator between them.
When they are not the same age, the charge rates will be different on them, and at some point, one battery might drag the other one down. Since you found one bad battery, the battery you think is still good likely has been trying to recharge the bad battery. If done to excess, this could have also damaged it.
For this reason, the conventional wisdom is to always replace both house batteries at the same time.
Have you tried to operate the Schwintek slide with just the new battery as a test?