It's a shame he's sad but you're only hearing his part of the story.
A decent, reliable campervan can be quite expensive and perhaps she's thinking that you can get a lot of B&B holidays for the cost of a depreciating asset that requires spending money every year on tax, insurance, breakdown/recovery cover, and maintenance. My friend spent quite a lot of money on a second hand campervan and almost immediately she had to have repairs done. And I suppose they use more petrol and are more difficult to manoeuvre than a car?
I really can't see why he can't just do a B&B holiday. If he can afford a campervan he can afford that.
Perhaps he feels a bit pushed out by the grandchildren and if he is helping with them a lot perhaps, as he says, it is too much for him. But those are separate issues and, if he does feel that way, he should maybe discuss it with his wife.