I am a firm believer in caravanners having a Multimeter and being able to use it for basic 12 VDC tests
Checking the Battery voltage
Checking Fuses
Just having one with them could allow the forum to "talk" them through some simple checking tests.
I will look for as many e-books on the subject as I can.
http://nadp.sws.uiuc.edu/cal/PDF/MulltimeterUse.pdf
Multimeter and free E-Book
http://www.satcure.co.uk/accs/page6.htm#multim
https://www.youtube.com/watch?v=JF0n-Dw3Dt8
http://www.allaboutcircuits.com/vol_1/chpt_3/9.html