I meant to update this thread earlier.
As I posted earlier I don't think the Progressive Dynamics 4000 series would have fit in the old space, and I really didn't want to put it anywhere else. The 4000's want more than 8 inches of height (on the front at least). That's just too tall for that spot.
I ended-up buying a PD4645. The 4600 series are upgrade kits for a specific set of older converter chargers. Being an upgrade kit, it splits the converter/charger into two separate pieces that end up connected together inside whatever converter/charger is being upgraded (it replaces the guts of these older converter chargers). Obviously I wasn't upgrading anything, so it seemed wrong, but looking at the measurements on the Progressive Dynamics web site, it looked like the main part of the upgrade kit would fit in that space below the furnace and then I could fit the other piece of the upgrade behind it where I could get to it from the access door on the side... so I went for it.
It worked perfectly. I had to slightly enlarge the hole where the old converter was in order to slide in the new converter. The depth was exact - the back wall of the converter sits against the wheel well. The front sticks out from the wall an eighth to a quarter inch, but it appears, from where the mounting holes are located on the front, that it was meant to do that. I covered the front with a baseboard vent. The vent has some depth to cover the sticking-out converter. Home Depot had the vent. It looks great - like it was meant to be.
That was only part of the install, though. The other piece of the upgrade kit is a circuit board for the 12V connections and fuses and for the battery connection. I mounted that board to a piece of wood which I then stuck to the wheel well behind the converter with epoxy. This way, I have access to the circuit board from the door on the side, beneath the closet (the one right at the trailer entrance). You need that access to make your connections, replace fuses and to see the charge status light
. There's a green LED on the circuit board that lets you know what it's doing to your battery (it blinks slow or fast or not at all, depending on the charge status).
I'm super-happy with how the installation turned-out and it works really well.