I found a nice Voltage Monitor at WalMart today in their battery section. I do not like LCD readouts because readability is so variable depending on ambient light. This is easy to read, as shown below in full sun. Cost me 16.88, and just plugs into the power socket. Of course, it can be hardwired also by cutting off the plug.
It should be on a switched circuit, since it draws about 0.1 amp. I am planning to install it with a SPDT center-off switch so that I can monitor either coach or chassis systems.
Shown below is LOW Volts warning. It also has one for HIGH volts.
2008 Itasca 37H
2011 & 2012 Len & Pat's "One lap of America"
27K miles & 41 states in 13 months
Yellowstone Lake 6-1-2012