Some good questions so far, so I will give a bit more info. in this post.
Oldme covers why I chose to do my own thing. The pre-made transfer switch is nice, but has half the capacity of what I built. You also can't see what is going on inside the guts of the box. The inverter and relay box I used are under the overhang of the corner bed, so they are totally out of the way AND completely accessible. With the open frame contactor in a nice big box, I can remove the box and look inside to diagnose anything that could go wrong. I even ordered a spare contactor because they were so affordable ($29). If I decide to power higher draw AC devices outside the coach (microwave maybe?), I have some growth room too. I work in IT and a tough lesson you learn is that you need to future proof every installation as much as possible.
jmaotto - You ask a bit of a loaded question. If you get your fridge as cold as it can go, then cut off power and keep it closed, you could go all night and have safe food the following morning. If you are asking how long did my fridge operate off the batteries this weekend, my guesstimate is about 9 or so hours of run time on what I believed to be pretty much full batteries. However, they drained down to under 12V and you really want to try and keep to no more than a 40-50% discharge.
I am happy overall with where I ended up, but today I was looking real closely at load/run time ratings on several batteries. My current coach battery is a 95Ah wet cell with the little sealed battery giving another 18Ah for a total of 113Ah. I can buy 105AH batteries (same brand as the coach battery) for $99. Provided I can fit a pair of them under the chassis somewhere, I could be running around 323AH of battery capacity for another $250. I need to install an ammeter to be positive how much energy the fridge uses, but researching battery web sites I find that an "average" fridge running on an inverter would use up about 160Ah per day. If that is a close number, then having 323Ah of battery would be about right for a 24 hour cycle.
Now that I can power the fridge, there is the issue of charging. With a 30 year old RV I seriously doubt I have a really powerful converter and you need BIG power to recharge your batteries in a reasonable amount of time. With a 100 amp converter, it would take about 2 hours to fully restore charge from 50%, so an upgrade would be prudent. The Powermax PM3-100 is looking real good at $153.
I started this whole thing because the cost of replacing my broken 3-way RV fridge with 8 cu/ft of space starts at about $1500. The real cost of replacing with a residential, 120V only model will end up at about $1200 total. The benefits of going this route are not just lower cost though. The fridge I bought is bigger inside than the RV model (9.9 cu/ft), I have more propane to use for my heating/cooking, I will have an upgraded battery charger in the coach and I have inverter power enough to run other power hungry gadgets or charge sensitive electronics.
Provided you can do the work and have space for batteries, I really do feel that using a residential fridge with an inverter is a good way to replace a failed factory unit.
Jose
