RV Repair: rv house batteries, lead acid batteries, rv batteries

Question
2000 itasca horizon 36'
3 new interstate SRM24 batteries
freedom 15 invert/charger
heart interface remote

1: the batteries seem to die quickly under light use.  13.2v - 11.7 within aprox 2hrs. is there a test i can run to determine if the system is okay? or if i'm expecting too much? such as, turn on a 100w light bulb through the inverted 110v sys for __ hrs. then the batt voltage should drop from 13.x to 11.x?

2: i have 3 house batteries, how should they be hooked up? LOAD; pos to #1 batt and neg to #3 batt and just the opposite for charger?  i thought i read that somewhere but it didn't make sense since the resistance through the cables is very small so should be the same electrically no matter which batt terminal is used.

3: i recently bought a used coach and all the batt cables are wraped. i suspect some corrosion. how can i check without unwrapping.

Thank you for any help.
Neal Pyke
anacortes wa

Answer
Hello Neil:
I will try these one at a time.
1. Thing to remember about inverters is that since you are kicking up the voltage by a factor of ten you increase the amperage draw on the batteries by a factor of ten also. For example your tv draws about 300 watts of power, roughly 3 amps at 120v AC but the current draw on the batteries through the inverter is about 30 amps. Batteries can not support that kind of load long without a voltage drop. Normal lead acid batteries have about 180 amp-hours of storage and start to decay voltage almost linearly with the amount of storage left. If you try and pull out 90 amp-hours of storage voltage will drop to 6.0 volts. Ofcourse your inverter will cut off for low voltage long before that. Maintenance free batteries, marine and RV batteries maintain voltage longer but drop very quickly when they do start decaying. A 100w light bulb draws 10 amps from the batteries. Make sure you are not drawing for the refrigerator or water heater those appliences can draw up to 100 amps from the batteries and you will drain them in a few minutes.
2. The three batteries are hooked in parallel, that is all the positives are hooked together and all the negatives. It does not matter where you hook the load or ground, it is a self correcting system in that the highr voltage battery supplies more voltage always. The positive from the charger hooks to the positive on the batteries and forces the current into the batteries to take the lead out of solution and deposit it back on on the batterie plates. When you discharge the batteries the lead goes into solution in the acid from the plates.
3. I have very seldom seen corrosion inside the battery cables. Most corrosion starts on the terminals and works it's way into the cables a little over time. Corrosion inside the cables is always accompanied by a bulge. If the cables look good at the terminals they are probably ok. You could check for a resistance but it will probably not tell you much. Probably a better way is to check for a voltage drop at the load end under load.
Bill