Let me clarify a bit.
The separator/combiner/solenoid/relay (gets called many things) does three things basically.
1) Makes it so that when you are camping and drawing on your house bank (batteries), you are NOT drawing on your start battery. It does it automatically (separates).
2) Makes it so that when you are driving, the start and house bank are made into one (combined). This allows the Ford alternator to somewhat charge your house bank (only somewhat as the alternator is essentially a "stupid" charger). Certainly does help to put some juice in though.
3) With appropriate wiring, makes it so that you can purposely combine the batteries to "jump start" yourself. The only purpose this serves is if your start battery goes dead, rather than get out jumper cables and hook them to someone else's vehicle, you can jump from your own house bank.
You CAN do all of the above with one simple, robust, relatively inexpensive switch (has been done this way on boats for eons), BUT you have to do it, vs. just having it all happen behind the scenes. Personally, I prefer it this way, but I'm sure I'm in the minority. I do have a double set of LED lights to help me out (red flashing in coach and green steady in cab when combined), and I also have a system that is not in dire need of being either combined or un-combined.
If I were on a budget and had an ailing separator and mystery switches, would strongly consider a switch, but you'd have to be the type that doesn't mind switching it "off" when parked/camped (separated) and "on" when driving (combined). Or "on" to jump start (combined).
The Blue Sea 7622 does all of this rather elegantly and automatically (with manual jump start combine), but costs a bit more and is a bit less simple. A fine product though.
2) The "battery disconnect" switch you have performs a different function (I assume, but may be wrong). It's a bit ambiguous as shown.
a) What I think it may be doing is serving as your house battery switch. What this does is disconnect your house bank from the house loads. Nothing to do with the Ford/cab/start battery.
b) But it also could be "disconnecting" your start bank from your house bank, maybe.
I'm happy to try to help you work through it, if you can report back in with clues. There could be some relatively budget solutions (or at least "solutions" that are at least an improvement on what you have now, if not the ultimate).
I should also warn though that of course I can't see your rig, am only an amateur, etc. Just so you know.
BG
PS: This is the switch I would recommend if you were going that route. It's the Blue Sea 9004e. You could use a couple of other options too, that cost slightly less, but I like this one because it has two little "field disconnect" terminals on the back that make it super easy to wire in little lights to remind you when you are combined or separated.
If your rig is wired up anything like mine was, what I would probably do is first, take the two main wires that are on two different terminals on the 1315 separator (which may be recalled anyway) and put them on one terminal together. This essentially turns it into just a power post and takes it out of the picture.
Then, where the alternator wire comes into your house battery bank, take it off the battery terminal and put it on one side of this switch. Then a small jumper wire from the other terminal of the switch to where you took the cable off the battery post. Now the switch does the separating and combining. I have mine where I can reach it from the driver's seat, which is handy, but I'm just mentioning this as the simple/cheap option. I'm basing this on your rig being similar to mine, but not sure if it is.