Tankless water heater options on a 35' HR

Tankless water heaters require a certain GPM flow rate to work effectively. CG's and water pumps are pretty inconsistent and I would think would impact performance. I always scratch my head on how or if newer RV's have solved this problem. If my water heater crapped out I would apply the KISS method and simply replace it with the same kind. Why invite problems? We have one in the S&B and despite consistent flow rate, you invariably get the cold water sandwich. I actually had to install a 2.5 gal under the sink tank water heater to feed my dishwasher due to the latency when it cycled of getting nothing but cold water from the tankless. We had an electric tankless in our old S&B and it required 3 dedicated 50 amp circuits (one for each burner). Granted an RV unit (if an electric one exists for RV's) would be smaller, but it's a huge amp draw to heat water from 50 to 120 degrees in the span of a few inches. I think this would be an expensive undertaking to get worse performance than he had with standard tank just to try and get a few extra inches of storage space.