Novice Qs re. cabin's small solar system and controller

So, the guys went up to the family cabin the other weekend, and since four of them now have CPAP machines, the backup/nighttime battery bank died out partway through the night, every night they were there. (Normally, the batteries can keep the fridge and a single CPAP going, through the night.)

That spurred the guys to check the solar setup; whereupon I was called, because they wanted me to order them a new solar panel controller. (I have some experience with solar and batteries and such, but primarily only very simple setups for camping, portable use, etc.) Since I live four hours away, and had nothing to do with the original setup, I asked for all the possible info--and pics--they could get me... Unfortunately, I did not get much(!); but here's what info I do have: 

Solar panel(s), Arco M-53. I do not know if this is a single panel or group. (The guys say it looks like one narrow/long panel, but don't seem very positive.) Arco M-53s seem to be from the '80s, and this setup had originally had come off a camper setup. From what I can find on old forum posts, I've seen them described as 35 watt, 43 watt, or 52 watt; so I assume it's/they're roughly in that range. The guys drew a 17-something-volt reading off the panel(s).

Existing controller is an M-8. From what I can discern, it seems they tested this by disconnecting it from the batteries, and trying to read the voltage on the output (either 1.3 or .3V, depending on who I ask), with no other readings done. It doesn't sound like they ever checked it connected to actively charging/drained batteries. (And hope no damage was done by disconnecting it/reconnecting it that way.)

Battery bank is a group of four deep-cycle batteries, presumably in parallel, unknown amp hours. (Had verbally steered them away from replacing with further starter batteries, or lithium/LiFePO4 batteries--very cold climate--last time they had bought new batteries.)

These batteries ALSO get charged via diesel generator, which is manually turned on when desired, to power the cabin. Presumably, the generator/cabin power is then fed to the batteries via a battery charger (unknown specs/amps), and then goes from the battery bank out through an inverter (also unknown specs), to an independent outlet for keeping the fridge/freezer going at night, and powering any other crucial items, such as the CPAP machines.

(The generator is the cabin's primary source of power. Typically, it is run for periods during the day, and shut off at night.)

(Unrelated, there is evidently also an automotive-store-type solar trickle charger (unknown model/specs) on the generator's battery. This has no separate controller, although it hasn't seemed to have overcharged/caused a problem, in the past.)

Bottom line: Assuming the cabin's battery charger is sufficient and working fine (worth looking into what that's kicking out, and how long the generator needs to be running in order to fully charge the batteries), I don't know whether it's really making a huge difference whether the cabin's solar controller is working, or not? I'd think the current solar setup isn't capable of doing much beyond keeping the batteries topped off when no-one is there; and that the generator/battery charger is doing the heavy lifting, when people are there? (The CPAP solution may be to either up the amp hours of the batteries, if needed; and/or I could just send along my own charged LiFePO4 batteries and an inverter, for them to utilize independent of the cabin batteries, those rare times so many CPAP users will be there, at the same time.)

However, as far as potentially ordering them a new solar controller, I could: Either have them properly retest the existing one, first, next time they are up there (and get me more info, in general); OR just go ahead and maybe get them an overkill controller (perhaps a 20 or 30amp PWM with reverse current/backfeed protection?), which then may also be able to be reused if they ever change out/increase their solar panel setup; and then also add a fuse between the controller and batteries??

But I am totally out of my knowledge/comfort zone, here... What would your recommendations be, and do you have any additional concerns for this particular situation??? More fusing? How about grounding? Backfeeding concerns, or any further concerns with the dual input (solar and charger/generator)??? Etc.?

This cabin is remote and hard to get to, and is mostly vacant, so I'm very nervous about any potential for fire or etc... I really appreciate your thoughts and help!

(Sorry for the long post!)