Assuming that the pump and it's control is not the issue, I would look to the possiblility that there is air in the system on the pressure side of the pump. I had air in one branch that would make the pump surge until I opened the faucet to let the air out. I always replaced the pump that came with the RV with a variable speed pump that made the flow more even and quiet as well. The stock pump was extremely noisy and cycled on and off rapidly. The pump that came with our 2018 RV is a regular pump, but for whatever reason, the new design is so much better than in the past. I did have a surging episode when air got pulled into a hot water line to the shower. It caused not only the change in flow, but it made the tankless hot water heater crazy because it senses flow and then turns on.
