Less than 13.2 volts while plugged in says your charging system needs looked at.
BTW get a portable charger on those battery asap before they get sulphated while you get the converter sorted out. If spending money get at least 5 amp rated 10+ amps is better.
tbax46 wrote: The meter on my leveling system shows 12.1 and my handheld tester shows 11.8. Should there be this difference?
To answer this question specifically ...
MAYBE ! First, meters are not perfect. The less expensive they are, the less accurate they are, but two different meters should be within a few tenths of a volt ...
... if you are measuring the voltage at the exact same point! The size and quality of the wiring between two different locations can make a big difference in the reading !