15w40 and whatever FL-1A size filter you choose. It wont care. I tried an oversize filter (FL299) on my Sis-in-Law's that is similar to yours, but there was a hard line for the trans cooler in the way. YMMV.
Check the tires, campers usually get replacements from age, not miles. The inner rears are a pain to fill. NAPA has a kit for duals to tie them together in a common fill point.
LED bulbs to replace the hot incandescents in the overheads. Amazon or PPL in Houston.
The front window in the overhead bunk is a leak point. If you see any intrusion, pull the whole thing out, reseal with 100% silicone (aquarium rated) and reassemble.
That thing is a time capsule, lots of campground cred there. I would change carpet though. It takes a beating in a camper, and years of dirt and smelly feet along with the inevitable pet schmutz that you don't notice until you start pulling it up.
I've been watching (and slightly hooked on ) a lot of "The Chick's Life" on YT lately. This family sold it all, bought a rig, and document their adventures full timing. Lots of ideas for places to go, what to do, and adventures. Light, family oriented, and done better than several Travel channel shows.
Good job OP.