Susan, your husband has a generally valid point, however you only need ONE exception.
Find ONE eager seller who is moving UP to a bigger unit, and needs the pad space or the down payment or is just organized and wants to do the changeover in a logical order.
Find ONE dealer who wants to sell that 25-30 foot Airstream, and doesn't want a good newish used unit on the lot interfering with the fast sale of a NEW unit, and there you go.
Find ONE couple who love their little Airstream but have lost jobs, have had triplets, have filed for divorce, or have health problems that preclude traveling... and you're rescuing them. (And if you pay $500 too much for the trailer it's a mitzvah.) Would you pay $500 more to go camping all summer?
So seek the one, find the one, become one with the one.
Just keep reminding him that you agree "we probably won't find it, but if we don't look, it's definite we won't... and half the fun is looking anyway"