I'm honored and flattered, but, That is throwing a lot of responsibility on me. I was hoping to present you with the info to make the best decisions for yourself. I have a tendency to make things more complicated than they need be when attempting to explain things. Do not forget this.
While I think the Sears DHP group 31/Odyssey is one of the best batteries available, I can't say for sure it is the right battery for your engine, and what your vehicle's charging system can put out. If you draw this battery down deeply, it needs to be recharged at a high rate. If your charging system cannot meet this high rate, then the battery will not perform as it would otherwise, and a lesser, less expensive battery would be a better choice. How can we know whether your Vehicle meets this high rate? We can't as you do not have the tools to measure it, or the knowledge to use them if you did, among some other factors.
Also the manual switch for battery isolation is not idiot proof. If one forgets to turn the switch after shutting down the engine, then one can draw down the engine battery. If one turns the switch to off instead of Both with the engine running, one can fry their alternator. There are other methods to isolate engine batteries from house loads that are automatic, and Idiot proof such as a Blue seas ACR.
http://www.bluesea.com/resources/1366.
One can also use a simple continuous duty solenoid triggered by another circuit that is live only with the engine running. It connects the batteries with the engine running, separates them when off.
There are other methods and products to accomplish essentially the same thing, all with benefits and detractions. I use a manual switch. It does not mean it is the best way, just that it works for me and I am careful with the responsibility.
What is most important is to develop the basic understanding so you can accomplish installing these things yourself. Solenoids fail. Would you know how to use a digital multimeter to diagnose? or just throw your hands up in the air, declare yourself worthless in such matters and hire someone to figure out what is not right somewhere. Hard to learn by words on a computer screen too. Hands on is the better way. Getting a 3$ centech digital mulitmeter and placing the probes all over the battery terminals and other wires measuring voltage can give you a better understanding of your system than anything I can guess at or try to relate to you from here.
Living from battery power can be a bit complicated, and beginners usually destroy batteries by over discharging and insufficient recharging, and even me, as long as I have been living on 12 volts, make mistakes more than I care to admit, and I am still learning. I brought up the sears DHP group 31 more as a contrast to the 8d battery you were shopping for. I think you could go get two group 24 marine batteries from wal mart and place them in parallel in one of your current battery trays for half the coin and most likely still have more than enough CCA to start that cat diesel. These won't be as difficult to replace when you almost inevitably kill them by running them too low and not recharging them properly
A couple things to take away from this, is that golf cart batteries are the best bang for the buck for house electrical duties. The most resilient to overdischarge, but not immune to it. The most tolerant of improper recharge, but not immune to it.
The engine starting battery needs to meet some CCA number which it seems there is no definitive answer to online, that we have been able to find anyway. I am guessing that 1150 CCA is more than enough, as 1150 CCA is a LOT. I think a definitive recommendation by somebody with experience with your specific engine is the best course of action. Just because you have 2 8d's which appear to both be starting batteries does not mean the engine requires 2 of them for that task. You do not even know if both are indeed in parallel or if one is for the house and one for the engine. A simple check with a voltmeter on both battery terminals, engine running would determine so.
if you can successfully remove all House loads, but the engine starter from the engine battery, then you can just get a Starting battery, which are generally much cheaper than marine batteries or true deep cycle batteries.
I am assuming the previous owner or your vehicle just decided to stuff as much battery as possible into the engine compartment and not worry about battery isolation. perhaps the previous owner just had some halfwit Bubba tell him this was the best way, and here look I just happen to have these 8d batteries sitting in the corner unused and barely ever used I'll give you a special deal on them.
Please don't just ask me to tell you how to spend your money. If I were there to inspect the system in person I'd present you with many different options, on what we can reuse, and possibly retask and what simple needs to be redone completely to best achieve your goals. I can't really do that over a computer screen, and you don't have the tools or knowledge to present me with what I need to present you with your best options.
Also do not rely on just my word. For example i do not currently have a Converter such as an Iota. Would you take my recommendation that they are a good charger/comverter/DC power source compared to the person who has been using one for years and has less than a stellar experience with one?
All I can do from here is list pitfalls to try and avoid, and the biggest one is finding some quasi Guru on the internet to tell you how to spend your money. Especially when one of the products is an expensive female dog if mistreated, like the Sears DHP battery.
I've had a lot of work recently and the only reason I've been this wordy on this thread/topic is because of the holidays and I decided I needed some time away from my powertools and petulant kids screaming at their mother in the background. I have so much work that I will not be able to devote as much time to further responses nor be necessarily be able to respond in a timely manner.
Things you should research to help yourself is how to solder wires, how to crimp wires, or find somebody who can do both whom you can develop a relationship with.
Copper and Lead are expensive, and you are getting ready to expend large amounts of money to acquire both.
If you are not going to be putting batteries into use immediately, then don't buy them just to have them, wait until you are ready to put them into use, as they have shelf lives.
