The Magnum display would be indicating house battery voltage.
The Magnum web site or a Google will find the documentation. You likely have the RC50 remote panel, maybe ARC50.
Yes, float voltage is typically 13.something - double check the battery type is cinfigured properly for your batteries.
As Twinbiat noted, ensure the battery temperature sensor is in place, if so, disconnect it at the inverter to see if causing a problem.
Magnum tech support is responsive and very good. If needed, they can step you through any configuration needed.
Vince and Susan
2011 Tiffin Phaeton 40QTH (Cummins ISC/Freightliner)
Flat towing a modified 2005 Jeep (Rubicon Wrangler)
Previously a 2002 Fleetwood Pace Arrow 37A and a 1995 Safari Trek 2830.