A few questions (1981 Pace Arrow, Dodge 440)

Does the Parking Brake Handle have a small cylindrical metal knob/handle? And a sector that looks like a kid's protractor? If so, it's what we had on our 1971 Dodge Van. I think the Handle lines up with the Cable when the brake is applied. If it's like that, you should be able to clamp the frame (where it would mount if the wood hadn't rotted away) in a Vise-Grip Pliers. Get the Pliers flat to the Handle and the Handle should just pull toward the Pliers (like using a Hedge Shears). The way it worked was the Handle would get past center and stay there till pushed down. That's a little different from the brake between the bucket seats of passenger cars. The car ones don't pull a full 90* to lock, and have a button to push to release it.
I've seen the Dodge Brake I described here on enough vehicles to hope it's what you have on your motorhome chassis.