Battery Volts in my Bucket-o-bolts

[HoosierDaddy]

My engine Batts were AWOL this spring. Interstate 8D's ...Each had a bad cell. Since I am able to buy from my local distributor at dealer cost and they were only 24 mo. old I took them back. Meanwhile I took the 2 good house batts (also Interstate 8 D's) and installed them into the engine start position. I would like to install AGM's but I'm not ready to spend that much yet. My Interstate distributor had two SRM 4D's ...Deep cycle that will fit into the same space. They made me a deal that I couldn't refuse so now I'm trying to figure out how long they will keep the 120V fridge running. Their specs are:
RC min.=390@25A (Does this rating mean they want at least 25A recharge rate?)
38 hrs @5A
11.5 hrs @15A
6.5 hrs @25A
I'm assuming those ratings are at 12V and would yield 22 hrs@ 7.5A average..
I'm also assuming that the fridge will consume 5 to 10 amp @ 120V so after converting to 120V would net approximately 2 hours. SSSoooooo having two batteries should last approximately 4 hours...???
Somebody please confirm my brilliance or gently point out my folly!!!!
Here's a link to the interstate spec page:

www.interstatebatteries.com/cs_estore/content/product_info/marine_f.asp

[prairieschooner]

Here is a link that you may find helpful;
http://www.westmarine.com/webapp/wcs/stores/servlet/WestAdvisorView?langId=-1&storeId=11151&catalogId=10001&page=Selecting-a-Marine-Storage-Battery

[fulltiming]

RC is reserve capacity. A fully charged battery will last for 390 minutes or 6.5 hours without being recharged with a 25 amp load before being brought down to 10.5 volts (for a 12 volt battery). This is lower than you would want to discharge your battery on a regular basis but you could in an emergency.

A 21 cubic foot residential side by side refrigerator uses about 780 watts of electricity at 120 volts (6.5 amps). Assuming a 94% efficiency factor for your inverter, at 12 volts that would suck up 1660 AHr a day if it ran 100% of the time. However, typically, a refrigerator runs about 33% of the time or about 553 AHr per 24 hour period. At higher loads, batteries don't hold up as long as they do at lower loads (as shown in Interstates chart of 38 hrs at 5 amps (190 AHr) vs 6.5 hrs at 25 amps (162.5 AHr). When the compressor is running you are pulling about 70 amps. Spread across two batteries that is still 35 amps/battery. That is one of the reasons that more batteries are good. If you could feed the refrigerator off of 3 batteries you would be pulling 23 amps/battery and the 162 AHr would apply. Since they don't show the hours of life at 35 amps we can only guess that it is likely less than 140 AHr or about 4 hours. With the refrigerator cycling (assuming the refrigerator is already cold and isn't having to run 100% of the time to cool itself down), two batteries should give you about 280 AHr or about 12 hours BUT that takes the batteries down to 10.5 volts and you REALLY don't want to do that. Realistically, the two batteries would likely need a recharge about every 6 hours. That is the reason that Newell's switch to residential refrigerators was matched with an increase in the number of house batteries from 2-8D's to 6-8D's and a 400 amp alternator.

[HoosierDaddy]

Very helpful..Thank you guys!
Dean