I see no responses to you so far so lets start the ball rolling. When on city water you say there are no problems so the issue is between fresh tank and high pressure pump line side feeding the hot water tank as a first WAG. Some clarifications though - what do you mean by:
“When the pump turned off the water was tank temperature! This would repeat every 10 - 15 seconds.”
When you are on pump water (from fresh tank), does the hot and cold water flow normally though other coach sink faucets, but just not at the bathroom shower ?
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