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That is too low on the LP pressure............it should be more like 12 WCI static which will give you about 11 WCI with about a 35K btu use that would be found in most RVs. Be sure that the burner is clean and that the installation tolerances are correct. Sometimes the LP flow can be impeded from debris inside the inlet screen on the gas valve that is left over from construction..............sawdust and the like. IF this is working well on AC mode then these would be the most likely causes of poor performance on LP. Hope that proves helpful here. Please get back to me once you examine these items if you need more as all follow ups are included in our fee.