The energy store in your Eriba, the 12v leisure battery,
as a minimum it powers your water pump and your
internal lights, if it goes down you will miss it but it is
almost always sadly neglected.
A Digital meter is very useful and more accurate than the
old deflecting needle type. The difference between a fully
charged battery and a flat one is only around 1 volt.
100% charge is 12.8v
75% charge12.5v
50% charge 12.2v
25% charge 12v
Discharged/Flat 11.9v
Don't leave a battery discharged for any length of time
or it will never fully recover.