If the spare is not a 40 amp, you should be able to find a 40 amp mini circuit breaker for about $10 at an auto parts or marine store.
Buy one that has mounting tabs like the ones on your rig and check the label to see if it's auto or manual reset. I'd probably use the auto reset.
I carry a spare 50 amp breaker for my hydraulics pump. Sometimes they get tired and trip in the middle extending or retracting a slideout. If you're shopping you might want to also get a 50 amp and keep it on hand. $10 can save you a lot of aggravation.