I feel your pain.... I have a 20 year old 420, and I think that I have spent about 500 a year for the last five just to keep it rolling....
Idea, but I am sure that you have already tried it... the grounds can be bad. Rework EVERY connection to ground (battery cable, engine mounts, starter and solenoid attachments. If the grounds on any of these are corroded, then you will not be getting full voltage where you want it.... Just a thought.... Do you have any ideas on "flooding Onans???"" (ha ha)
GOOD LUCK! I am so close to just dumping mine, but I just keep thinking that maybe this fix will be the last one....