A while back, I was soliciting thoughts/comments about the idea of using a single AGM8D for my start battery. By the end of the discussion, I had decided to go ahead with that idea, plus at the same time I wanted to update my OEM isolator panel. This is a report on the end result of those changes. Instead of adding to the old thread, I'll make this a separate post. If interested in the previous discussion, see link below.
Engine Start Battery Options
First stage: Upgrading our isolator panel.
Photos show our isolator panel, "before and after", and the new dash solenoid control switch. After cleaning up the old isolator panel, I installed a Victron Energy ARGOFET (low voltage loss) isolator with two output posts rated at 200 amps each. I replaced the old boost solenoid with a Blue Sea Systems ML-Solenoid PN 7701. I also installed a new Cole Hersee 24213 200A (continuous duty) aux start solenoid, and 3 fresh 15A Buss auto-reset circuit breakers.
Victron Argofet Battery Isolator
Blue Sea Systems 7701 Solenoid ML 500A 12V RBS
Amazon.com: Cole Hersee 24213 12V 200A Continuous Solenoid: Automotive
After they were cleaned up, I was able to reuse the existing battery cables with the revised isolator panel layout. For full functionality, the supplied control switch for the ML-Solenoid requires a 3-conductor cable from isolator panel to dashboard. For this purpose, I originally planned to use the original BOOST control switch wire plus two of the "extra" spare wires provided by the factory, but I was unable to locate suitable small-gauge "extra" wires in the engine area. I know they are probably back there somewhere...but I couldn't find them, even using my signal tracer tool. I did find 3 unused spare heavy gauge wires (red, yellow and green) running from dash area to engine compartment, but they were way oversized for this job, so I decided to save them for some future project.
Instead, I figured I could run a new 3-conductor cable through the interior of the coach, following the existing factory wiring that runs on the floor along the wall on the driver side. It took some odd (sometimes painful) body contortions, and a little bit of (tedious) work with a fishing tape, but I got it done. In case anyone cares, the path I followed is as follows: Start under the dash, run along the side wall next to driver seat (under carpet edge), drop down behind and under the sofa, around the propane heater box, behind the under-sofa storage drawer, underneath all the kitchen cabinets, under the fridge and clothes washer, under both closet floors, through a under-floor wiring channel from closet corner to the electrical compartment at foot of bed, then down through bottom of coach (followed existing wiring) to the underside of coach, and over to isolator panel. Removing the bottom drawers in all the cabinets, and the "false" floors under the kitchen sink and in the closets, gives access to most of this route. The two tough "blind" spots were under the fridge and clothes washer, and between the under-closet area and the foot-of-the-bed area. In those spots the fishing tape was the only workable solution.
I mounted the new Blue Sea control switch in one of the unused blank switch positions on the dash. Perhaps you (like me) have never looked closely behind the dash switch panel. I was surprised to find (on our coach) 3 unoccupied rocker switch positions. The holes are already cut out in the aluminum base panel - they are just covered by the thin black cosmetic top sheet that you see when you look at the front face of the panel.. Once you decide which position you want to use, you can use a sharp pocket knife to trim out the mounting hole. Simple!
The switch is pretty cool. It is actually two "momentary" switches combined in one housing. There is a sliding safety cover that allows only one of the switch positions to be used at a time, and prevents accidental activation. When the switch is "ON" (solenoid closed - both battery banks are combined), there is a red LED warning light that illuminates to remind you of the battery status. Since the magnetic latching (ML) solenoid draws zero power when open or closed, I will be able to leave the battery banks connected for extended periods without a power penalty. Nice change from the original OEM design.
Second stage: Replacing old Red Top start batteries.
This is a pretty simple upgrade. The rack that held the two Red Tops is already sized and configured for a single AGM8D. The holes for the hold-down bolts are already drilled in the frame. I just had to buy a couple long (10") bolts, and a 2' length of angle iron to secure the larger battery. I ordered the battery at a local O'Reilly's, then drove the coach over to their store. I removed the Red Tops in their parking lot, and a couple of the young clerks brought the new 8D out and slid it into place for me. I positioned the battery with the posts facing "inboard", so the existing cables would reach. They had plenty of slack - I hooked them up with fresh new military connectors.
Important Note: If anyone wants to emulate this battery swap, I have a helpful hint. IF your start battery compartment is the same size as ours, and IF it is in the same relative position on the coach, then before trying to install the big battery, remove the hatch locking pin! You know...the thing the hatch lock clamps onto. Although I carefully measured the opening, and was sure the battery would fit through, in the end it did not. It hung up on the CAC end tank, and the locking pin. After the 3 of us struggled with it for several sweaty minutes, we realized the pin had to come out. So the store guys waited around while I scrambled to extract the pin. After the pin was unbolted and removed, the big 8D battery slid into position slick as snot. Learn from my mistake!
The AGM8D spins our old C8.3 engine over easily - feels and acts much stronger than the 10-year old Red Tops. The store accepted our 2 old batteries to satisfy the "core deposit" and refunded me my $22. EASY! 
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Topic: '93 U280 Isolator Panel Upgrade