I don't know that this is particularly helpful, but FWIW, when we're discussing this kind of big decision, we often find it useful to do a full cost comparison together. Ideally at a time when we are not particularly stressed, and can maybe enjoy some hummus and snacks while we jot down figures! Total up all the costs (or make reasonable estimates) for each option -- cost of each vehicle, insurance, est. cost of gas/year, approximate costs of conversion, approximate costs of the hotel rooms he'd otherwise have to pay for (minus cost of a campsite, if that's a consideration), and so on. It sounds like there's a pretty clear financial difference, but sometimes it's a lot more persuasive to see it all broken down and tallied up in one place...it helps us bring it down from the "wouldn't it be great" fantasy version and into the "oh, that's a big commitment" reality version.
We also try to list & discuss the less quantifiable, but very real considerations. It sounds like one of your significant concerns is family safety, while your partner may want to include something about the value of having a project to work on (or maybe something that feels like "home" when he's on the road? It would be helpful if he could explain where his itch is coming from!). In general, we approach this as a question of trying to make sure we both get our needs met, rather than a zero sum game.
So, for instance, if one of the less money-based desires is to have a way for the whole family to go camping during shoulder seasons, an alternate way to do that would be to purchase some extra camping gear (four-season tent, etc.). Or if it's a need to have some kind of project to work on, maybe there are other ways to scratch that itch too. And of course, if there's a way to make the van meet your safety standards, it would be good to know.
Lastly, we often try to acknowledge that no decision is final. No fancy camper van now does not mean that a van conversion isn't still possible somewhere in the future, when the time is a little better.