Back to the original posters problem. One other thing to check is where the negative battery cable connects to ground on the RV chassis. I had an older motor home that had an intermittent starting problem. After replacing the solenoid, cleaning battery cables at the battery, I found that the chassis ground where the cable connected was dirty and rusty. I cleaned that up and no more problems. It has been a long time ago and I am not sure but I may have even replaced the starter before I found the actual problem!
Good luck intermittent problems can be hard to diagnose!
Ed
P.S. When it wouldn't start, if I kept trying and "clicking" the key it would eventually start. It did not fail to start often, just occasionally.