camera bag

You should check the website for each museum you plan to visit. Most museums will not allow you to wear a backpack on your back. Sometimes you must check the backpack in their cloakroom. Sometimes you are allowed to carry the backpack.

Maybe a camera vest?

I agree with Laura. The rules vary for every museum. Most of the time, though, they consider a camera backpack the same as any other backpack. Your husband should be prepared to check his camera backpack at the entrance.

Usually I carry a small, 5 liter-ish camera bag. Even then, I have to check it at the entrance sometimes.

Depends on the museum. A camera briefcase or shoulder/cross-body bag would be more likely to be allowed in. It’s the “backpack” part, not the “camera” part that is more likely to be an issue.

Once at the Barbarini Palace in Rome they thought my camera bag was a purse and were going to make me check it. When I told them it was a camera bag, they told me it was okay to keep it with me. There is almost no way to predict the rules. Just be prepared to check any kind of bag at a museum.

My husband wore a small to medium sized sling bag in many museums with no problem. It was worn more forward and to the side. Size is bigger than a Fanny pack but smaller than a backpack. Not ideal for extensive camera gear but how many lenses do you need in a museum?