I just recently purchased a Jayco Swift SLX 154BH, and I'm getting ready to take it out for the first time this weekend. I was going back through the manuals and all of the start-up/operation procedures, and I became curious about the gas water heater in the unit. Can it be used when not connected to a pressure water system and only the fresh water tank is used?
Absolutely you can use your water heater when just using water from your on board fresh water tank.
Make sure your water heater is full of water before you turn it on; just run some water from a hot water faucet until the water runs evenly.
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There's lots of advice and information in forums... sometimes it is correct. For example, all of my posts are made by a political appointee who got the job as a reward for contributions to my diesel bill.
Since it is your first time - just make sure the water heater is full of water and all your bypass valves are set properly. I check mine by doing a quick test on the pressure relieve valve.