RE: Atwood water heater drain plug tightening
As another source of supply, I found genuine nylon plugs at Tractor Supply in their irrigation supply section. They were less than $2 each and work great.
RE: Toilet Fills with Water Pump On
I had a similar situation and found that the valve was fine but the foot pedal had begun to break where the "stop" is which let the valve remain slightly open. I managed to improve the pedal with a small block of wood to provide extra support for that area. Has been fine for several years now. You might want to pop the pedal off and take a good look at it.
RE: Hot water tank drain plug
I found a replacement nylon plug for my Atwood at Tractor Supply for a VERY reasonable price - under $2 if I remember correctly.