Generally speaking, you can't get contents insurance on a vehicle (but I don't know about RVs like motorhomes). Even if you have Comprehensive insurance on your van, it doesn't cover the contents. Usually, it doesn't even cover things that were added to the van after the van was bought, like a sound system. If it's stolen, too bad.
At least with this, I can understand why the ins. co. don't do it: people lie like rugs. They would lose something, claim it was stolen, and expect the insurance co. to replace it, over and over and over. Insurance companies are in the policy-selling business, NOT in the claim-paying business. And if you could get it, and did use it, your insurance premiums would likely go up afterward.
If you have some things that you would hate to lose, or are somewhat valuable, consider a very small rental storage unit. The smallest that I've heard about is 5x5'. Prices vary as to area (cities are higher, small towns tend to be cheaper). If you're talking about things like jewelry, consider a safety deposit box where you have your bank account.
BTW, you're going to get a lot of advice from people who think you should get rid of everything you own. Don't pay any attention to them, follow your own lights. If you eventually decide to unload the stuff you really don't use, fine. But consider that many people who live in vans didn't have much of value to begin with, and they would like you to be the same. If you put stuff in storage for $100/mo, and decide to get rid of it two years later, you've lost $2,400. You knew that going in -- it's no surprise. You paid the price to not have to agonize over getting rid of it. Things change. You change. It's called Real Life. Do it your way.
