12 volt water pump runs in bursts on hot side only

Sounds like a blockage somewhere
For a test drain the water heater
Open a hot water tap and turn on water pump
If it does the same thing blockage is on the input side to heater
Possibly a strainer ?
If the it happens after the tankless fills up
Blockage is on the output side of heater going to RV water system

I know that doesn't sound right

It just might be that you need to replace the water pump

Could be flow has decreased thru/from the pump