Thinking of replacing the vehicle battery, or leisure battery?
Assuming you mean the former, the chances of it having died are likely down to it having been sat redundant whilst the seller waited for you, the buyer, to come along. Happens all the time with vehicle sales. So it’ll want a charger with a decent charger (like the Victron 12/7, for example) and not just starting up once a week (as smart alternators don’t work like traditional ones do).
Opening and closing doors, using the radio and generally being in and out of the van whilst camping runs down the vehicle battery, so when camping for a few days, either hook-up and plug in your vehicle charger or make sure you have solar coverage.
Your leisure battery may have discharged your vehicle battery in those two days, too, if you have a traditional charging system installed over a DC to DC charger.
Another thing is the vehicle battery itself. Early vans came with Moll batteries, which depleted quickly. Varta replacements were fitted under warranty but there are still issues with them, as with any battery, if they’ve been allowed to go completely flat. There’s a thread online about replacing your starter with a bigger battery, but rule out what might be discharging it first.
Come back to us with photos / details of your setup and we’ll be in a better position to help! Oh and welcome to the forum