OK, following up with some info here, in case anyone (or myself) find this useful down the road.
Just finished putting the rudder assembly back together. 1993 Prostar 190.
Previous symptoms - couldn't seem to get grease into the rudder box zerk. Certainly couldn't get any grease to come out of the weap hole on the bottom of the assembly (just above the rudder.) Steering also seemed like it was a bit tighter than it should have been. Don't know if previous owners ever did any maintenance on the rudder.
1. After I removed the steering cable from the tiller arm, the rudder seemed to move very smoothly - no obvious issues.
2. Took everything apart, nylon washer at the top between the rudder box and the tiller arm. No nylon washer (at least on mine) between the rudder body and the rudder box on the bottom of the boat. What little grease there was in there was pretty nasty. I think the reason I couldn't grease it earlier was probably a tight/corroded zerk - or the grease was just so caked that the new grease couldn't displace the old grease, so no new grease could get in there.
3. Use a little engine degreaser, some rags and a toothbrush (don't worry - one of my kids - not mine ;-) ) and cleaned it all up.
3. Removed and replaced the zerk fitting. My size: 10mm x 1mm. Found at a local hardware store for about a buck. Didn't have any luck on these at auto parts stores.
4. Removed and replaced both upper and lower o-rings located in the rudder box. I just matched up closest size I could find at the hardware store. Went with 1-3/8" OD x 1-1/8" ID x 1/8" thickness. Seems to be a pretty good match.
5. Put it all back together and could actually get it to take grease via the new zerk. The tell-tale little stream of grease squirted out weep hole on the bottom of the assembly. So if nothing else, I got that going for me.
Honestly, I can't feel much of a difference through the steering wheel. But we'll see once she gets back on the water in the springtime. If I still have issues, looks like the steering cable will be next. At least I know the rudder assembly is good to go.
Now onto that next winter project - carb rebuild!!!