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Is the sensor getting enough flow where it is mounted? Mine is just one of those adapters, cut a section of the top hose out, put adapter in, screw the gauge into that.
FWIW. My rb30 takes forever to warm up. Just idling, even for ten minutes, only gets the temp up to 65-70. It did it with two thermostats I've had in it. A 5-10minute drive gets it up to temp pretty quick though, don't rely on just idling it to get it to temp.
Yours would take even longer as it's losing temperature to the gas converter as well.