Once again I must remind everyone that a voltmeter measuring battery voltage WHILE the coach is in use is NOT indicative of anything ...other than a dead battery OR a completely full battery plugged into shorepower!
You can be reading 11.5 on a voltmeter and actually have a REAL STATIC voltage of 12.5 OR you can be reading higher than you actually are. You can drive all day and take a reading at nite of 12.7 before starting to run stuff and then discover later that you never did have a full charge on the battery....or that your REAL capacity is no longer 100% of rated amp hours. It is better than nothing but gives you not a clue about your real state of charge...unless it is reading 10.5V
The OP should be fine with a couple of group 24's overnight with the furnace. This post is directed at those who think they are monitoring their batteries with a cheap solution.
The only REAL way to monitor batteries is with a monitor from Trimetric or Victron or similar and they couldn't sell them for $150 if they had REAL competition from $10 volt monitors. If you boondock a lot then you either should spend your $$ on a Victron or on the early replacement of batteries you "monitored" with a volt meter.