Two things... (1) The Girard Tankless WH that Jayco choose to install is KNOWN to have "short contact time" (meaning, the "flame" isn't "exposed" to the water tubing for a long enough period of time) and that it can - at BEST - only heat the water 20°-70° above what the "ambient water temperature" is. So... If you have VERY cold incoming water, your outgoing "hot" water may not be what you're used to in a traditional tank heater. (2) The Attwood brand is a better Tankless WH ONLY because the "contact time" is longer between the flame and the water tubing.
Both systems are COMPLETELY reliant on the ambient water temperature on how much they can actually heat. So... If you're in a "hot climate" and want HOT water, you may be able to use the water in your HOLDING tank (via the pump) for "higher temperatures" (the ambient temperature of the water in the holding tank is most likely warmer than the water from the underground system).
***ALSO***, the airflow for the Girard system is CRITICAL for a proper flame (blue v. yellow, you want a blue flame). Too MUCH air is actually not good for this system. Make sure the enclosure door is tight. Ours was a tad loose (from people trying to "hinge" the door versus lift it off the "posts"). Once I tightened the fasteners, the Girard worked fine - even for my wife (who was not pleased with her first cold shower).