Wow! A lot of ground covered in this thread.
Personally we never used our battery much in the teardrop, but it was always a guess whether there was much left in it, or if the little
solar panel I had was helping out at all.
Once we added the Airstream I became more concerned with power usage.
My solution was a Xantrex Link 10 monitor. It monitors everything in and out of the batteries so I know how much power I am currently
using (Amps) as well as Amp Hours, Volts etc. We I charge them up (shore power or via generator) I now know how much (AH) I have put back, recharge
rate etc... a wealth of info to know where the heck power is used and how much of it we gobble up each day.
It came in very useful to check usage from each light, appliances etc. Turns out the most power used was from the propane fridge! Dang thing drew a good 2A for the cooling fan, thats 24AH daily during a heat wave!
The Link 10 uses a 4 wire cable and a large shunt at the battery for sampling current in/out of the batteries.
See
http://www.xantrex.com/web/id/237/p/1/pt/5/product.asp
Paid about $199 if I'm not mistaken.