Obviously, this is not a Winnebago "warning" sign but one left by a previous owner. Who knows why it's there or what that previous owner did. So, we are little to no help with that.
Can you contact the previous owner or did you buy your 19-year old RV from a dealer?
If it was me, I'd hire an independent RV tech - one with solar experience to come and both figure out what you've got installed, how it is wired and what that warning note means.
It is possible to have bad or substandard wiring or other issues that can destroy your house battery bank and even be a fire hazard.
You should really check this out.
Do you have an Operator's Manual? The Ultimate Advantage motorhome was always a complex RV to operate.
Here is a link to an online PDF version of the manual:
https://www.winnebago.com/Files/File...4Advantage.pdf