That's reassuring on the Mains vs Solar charger competing issue. I've been thinking about it over the weekend and trying to find wiring information on the Elddiss Caravan its its a bit of a dogs breakfast without blindly drawing it out while disassembling it.
I went ahead with the Dometic PWM controller as I'd already ordered it when you mentioned I could get a good MPPT one for not much more. I fitted it over the weekend and also finished off the battery with a box from Burnsco screwed to the floor to sit the battery in. $35 for a large battery box with a strap was a great buy - Supercheap and Repco all over $50 and even then might not have been big enough the this battery.
It gives me more confidence to actually have a setting you can program for the battery type (Sealed / Flooded / GEL) that it'll be charging it correctly without over charging as I think the AGM tops at a lower voltage than Flooded. Its a nice controller as it has a backlit LCD screen thats nice and clear and shows Voltage and Amps coming from the panel and shows battery info. If I did do the load it then shows that and has timers to shut the load of X hours after sunset and things which some might find useful.
I'm not going to rewire as its too much of a pain without a good wiring diagram and I'm worried the mains charger is also the mains 12V power supply so if I put the load off the controller I'm not sure if it can also run off mains too due to the existing setup. If you're reasonably confident the current connections won't cause issues I'm happy - I generally can calculate what we're using and I'm going to replace the last few 12V ring tubes with LED so the only real draw will be USB charging phones and tablets. I try and do phones over night but tablets during the day with lots of sun.