Sitting on wet campsites this summer, I've been tempted. I've thought about this a lot ... however, it seems like duplication of space and facilities that you already have, especially if you're not buying something sizeable. And the challenges that exist on the California on a wet weekend, might equally apply on board a caravan or motorhome. Either way, it might suck. But it's an interesting idea and visiting a show and sitting in different products will probably answer many of your questions.
I used to tow a large Hymer caravan with a T5 Caravelle. That was a great outfit and, in a way, I miss it because I have such happy memories of the stays in the caravan. But it was a huge pain getting the caravan in/out of storage, setting up and packing away on-site, and thinking carefully about routes. Every time I set off in the California I feel like I've forgotten something because it's so incredibly quick and easy! Back when I buying the caravan, I did the (very good) C&MC towing course, and then drove to Germany to buy/collect it! However, it was my first foray into camping/caravanning and I couldn't shake the feeling that I wanted to be more flexible and more mobile, so after a couple of years I sold it to buy a new James Cook (2nd gen.) motorhome (also imported from Germany). I dearly loved the James Cook, especially being able to stand up and work in the kitchen while the beds were occupied, having a toilet/shower on board, oh man, it is luxury compared to the California but that had some limitations: the heating was hopeless (unlike the California), it was 3.3m tall (I got murdered on a long night drive by the French tolls), it definitely wasn't a daily driver (the huge, huge advantage of the California) and it was >3,500kg (my wife doesn't have that on her license). If Westfalia create a third generation, which they are rumoured to do, I would be extremely tempted.
Both the earlier caravan and the motorhome highlight the advantages of the California. Since getting the van in November, we've spent 25 nights on it and it has taken us to some amazing places, and been an incredible vehicle in support of our hobbies, particularly watersports. We've broken up longer trips with self-catering accommodation, and it feels okay to do that because the California is a daily driver and I don't have to justify using it for overnight stays (with the motorhome, I felt we *had* to go away to justify it being on the driveway). So I absolutely love the flexibility of the California, and being able to configure it depending upon what we need from it. But in my darkest hours, at 2am, gale forces winds, and I need a pee, I can feel differently!
I will be eagerly watching the discussion of Eribas! Jen - good luck with your Eriba trolling and let us know how you get on. Though if I bought an Eriba, I would have to get it colour-coded to match the Cali!