Good to replace the split rims - potentially dangerous and very few places will put tires on them, fix a flat etc.
Here's a tire diameter chart you might enjoy.
The tires on our 75 were 7.00x15s, 27.88 inches in diameter (by DOT numbers one of which could have been original when we got it a couple of months ago). But because there was tire contact with the lead edge of the wheel well, we replaced them with smaller.
Then the discussions will begin about truck vs trailer tires, preferred brands, "upgrading" to 16" wheels, etc.
My Father, a chemical engineer for Gates Rubber product improvement when he retired, was an avid RVer for over 30 years, his towables always had trailer tires and he never had problems.
So I figure if it worked for him ...
Tire life according to current sources is anywhere from 3-10 years depending on who says so, and if the tires are subjected to UV, unweighed and pressure reduced when in storage.