I installed four computer fans on the inside of the lower vent to force cooling air past the coils and up and out the top vent. It seems to have helped quite a bit. The second thing I did was install a computer fan inside, wired to the light switch (before the switch so it stays on all the time). That has reduced the frost building up in the frig to near "0". Of course, the freezer still needs defrosting.
Some newer coaches have two larger fans at the top of the frig at the back on top of the top cooling coils and connected to thermal switches so they only run when the frig coils are quite warm. They appear to be a common feature as I've noticed these on new frig units for sale at RV stores.
Finally, I'm amazed a 1979 frig is still working. You're lucky.