It is usually a water flow problem, most shower heads in RVs, are made to conserve water, so it could be the minimum flow at the tankless water heater is not being met.
Try an experiment, turn on shower to temp you would want it to be, turn on hot water faucet on sink to see if the flow is good enough to keep shower warm.
It could also be, amount of flow coming from hose, check the filter on the incoming water line, and the screen at the city water connection to make sure it is not plugged up.
