14.4v is a bulk charge voltage and should only be used when starting to recharge your batteries. Maintenance charge voltage should be down around 13.2 depending upon temperature and a few other factors.
re: "
which is recommended by Magnum Energy" -- on these sorts of recommendations you need to read the fine print. There are kinder, gentler, and safer ways to get long battery life for your RV batteries than to subject them to a full blown equalizing charge.
re: "
It gets cold enough in MN to freeze the batteries" -- if your batteries are fully charged and being properly maintained, freezing should not be a problem unless you get one of those winters where it gets to 90 below for a few hours or more. (see
Figure 7. Specific Gravity And Freezing Point. )
note that getting into hydrometry is hazmat territory. that is a good reason to avoid pulling off the caps of your batteries for anything other than checking electrolyte levels.