from what Ive read the on demand heaters seem to work great, I think the key though is sizing one appropriate for your pump, you can always decrease flow but cant increase it.. I'll be adding a pressure switched pump, surge tank thats sized to the heater that fits my little area.. I have no onboard water so it'll be pulling from jugs and draining into jugs..
here's the items i put in my cart like a year ago:
Buy SHURflo 182-200 Pre-Pressurized Accumulator Tank, Black: Tanks - Amazon.com ✓ FREE DELIVERY possible on eligible purchases
www.amazon.com
Buy SHURFLO (255-313) 1/2" Twist-On Pipe Strainer: Pipe Fittings - Amazon.com ✓ FREE DELIVERY possible on eligible purchases
www.amazon.com
Camplux 5L 1.32 GPM Outdoor Portable Propane Tankless Water Heater - - Amazon.com
www.amazon.com
SHURFLO pump is what I have in my home RO system, its really good quality.. tho expensive, was gonna get one for the trailer too.