There's no reason you can't use a 50 amp surge protector on a 30 amp circuit.
Does it have a diagnostic LED on it, that says the surge protection is working? Surge protectors do get "used up", the components inside take damage instead of letting it pass along to the RV, so they don't last forever.
It's electronic technology, and seven years old might as well be 20 years.
If it were me, I would just give it to a friend if they needed it and I didn't - an old unit is better than nothing, but it could be a false sense of security.