I would use an ammeter on my auxillary circuits ie fridge, camp lights etc this will give you a clear idea of when your battery is going flat.
for example if you have a 90 amp/hour battery (battery will be flat in 1 hour if drawing 90 amps) and your ammeter shows 9 amps your battery should last 10 hours. IMO that would be a better use for an ammeter in your patrol, and get it done by a professional. noting bugs me more than backyard electrics

Big Fletch am I safe in saying that you have Redone your fair share of someones dodgy electrics in your time.