I had an '83, 310 and after a front tire blew out, I found that a piece of rubber had hit and damaged the spedo cable which went through the left front wheel well. The spedo then jumped around and made a lot of noise. So, given the age of your unit, I would not be surprised if water had finally gotten into the cable and is causing it to bind. It is a bitch to get at to pull the cable out of the sheath and lube it, but that or replacement may be in order.
I would fix the vacuum blend door controls or the vacuum system rather than trying to convert them. The heater control cable most likely needs lube which is why it comes off the control lever. As you said, it is a 28 year old vehicle and everything needs maintenance.
Old motorhomes are a royal PITA to maintain, and costly. I eventually simply got tired of mine as there was always something wrong which needed repair and it cut down on my camping time. I loved it, but didn't use it for a number of reasons, including the never ending repairs.