Matching two different size tires as dual pair is bad news unless they are very close in effective diameter (and therefore the identical circumference). If one tire in a dual pair is larger than the other, it carries most of the load for that pair and is effectively overloaded. It will self-destruct real soon.
The 235/80 is going to be 4.25 mm more in sidewall height (188mm vs 183.75) which means an extra 8.5mm in diameter. That's about 1/3". That also assumes that both have the same amountof wear. If the 245 had 25k mile son it, its tread would be worn down somewhat, making it that much more smaller than the 235/80 spare, which is probably brand new.
1/3" extra in diameter isn't too bad - I've read that up to 1/2" can be tolerated. However, I wouldn't want to risk driving very far on it.
Former owner of 2004 American Tradition and several other RVs
Home is in the Ocala Nat'l Forest near Ocala, FL
Summers in Black Mountain, NC