Ashore I have an on demand propane water heater. House is plumbed like a boat w a Shurflo pump and an accumulator tank. Its a bit of a balancing act getting some of these heaters to behave, they are picky about water pressure and flow, but upping the pressure in my system helped. I think its at 45 PSI now and the system works well.
Check the specs on your heater for water pressure and gas pressure specs...they are picky about both. Unfortunately the documentation
is often poor so its a bit of trial and error to get it right.
Also check the details on your pump, some with internal pressure switches are actually fixed PSI and those adjustment screws may not do what you think.
...need that water heater now, DAMN its been cold on the Rio!