I would find a shop that deals with light and medium duty trucks. They should be able to find you the tires you need or find a proper substitute. It looks like you might be missing part of the tire code. But since basically you are looking for a tire for a "30 year old vehicle". That tire or size may be obsolete. You might take a photo of the tire (showing all codes) and contact someone like Goodyear support for options. You also might check to owners manual or newer vehicles have a sticker with tire codes to verify the original specs for the tires.
In Fort Wayne I'd talk to the guys at
McMahon's Best-One Tire & Fleet Services
(260) 483-8473
2828 West Coliseum, Fort Wayne, IN 46808
Mon - Fri: 7:00 AM - 5:00 PM, Sat: 7:00 AM - Noon
I see RVs in there all the time.
Good luck.