Welcome............. since your going Lithium I would advise a good read up on lithium Batteries, they require a specific charger the standard Wet cell/AGM/GEL charger won't cut it as for the original unit, its more than likely that black panel thats down close to the floor with a light switch marked battery next to it, its a Setek unit which the 240 plugs into and gives you 12v DC for lights and stuff when you don't have a battery and will charge the battery if its one of the ones mentioned above..... Have a look around your van, take pics, ask questions, pics help us to ID what your talking about, there should eb a folder which has the manual/instructions for all the items in your van if not go round with pen in hand and take down the details of things like Fridge model/brand, battery, HWS (if any) TV aerial, tow hitch etc, etc, then you can find online the manuals/ fitting instructions and ask questions here which we can answer without guessing what it is................
So ask away with heaps of details and/or pics, we do enjoy it and there's no dumb questions, some long winded answers and we may wander a bit but thats part of the fun..........