The Rambler was a popular Autosleepers conversion for a number of years, and I'd be surprised if it wasn't originally designed to charge the leisure battery from both alternator and hook-up - it was a pretty standard arrangement, even back then.
So certainly get the charging system checked out, for the times when you're on hook-up.
But since you need some "off-grid" capacity, for rallies etc, a solar panel would be worth having. I'm no expert on those, but with a small van I'd start with "What can I find room for?" then just get as much power as you can within that restriction.
I only have one leisure battery on my Transit, it's usually enough for several days without driving or plugging in, but recently I found it was struggling a bit if I needed the blown-air heater on a lot.
I bought a little fold-up panel online for about £25, the sort designed for keeping a car's battery topped up when not driven for a while. I can put it on the roof during the day, with an extra bit of wire so I can run it through the skylight to plug it in. It's nothing like as powerful as the ones designed for motorhomes etc, but so far it's been enough.