[busterbrown]

$10 volt meters (or the standard OE control panel "idiot" lights) are not reliable in an active RV coach. The disparities between load and resting voltages makes them absolutely useless.

The better route (and for not much more $$) would be to install a volt/amp meter with a battery shunt and measure current flow (amperage). Use the battery's amp hour capacity to determine what the state of charge (SOC) is at any given time, no matter the loads.

If the coach has the typically flooded lead acid battery, assume a 50% SOC would be the lowest a battery can safely be discharged to. For example, if the battery has a capacity of 80 amp hours, using 40 amp hours would put the battery at 50% (or empty).

Using the resting voltage, typically that would equate to 12.1V. it's just next to impossible to get an accurate representation of the resting voltage when the trailer is being used.