I'm making the assumption that this is your first experience with a trailer so I'm sorry if my post is a little long due to my explanation. I have a 2021 171RBXL and the outside storage above the water heater has a 'false bottom'. Mine is in 2 pieces with a cut down the middle. empty the compartment and lift out the left side of the bottom in the compartment. There you will see the red [hot water] and blue [cold water] pex plumbing pipes. There will also be valves, as indicated earlier in this post in a diagram. My camper is under cover right now, but if I remember right there are 2 valves. In the winterizing position they divert water into the hose that is connecting the red and blue lines... this is the 'heater bypass' position. I believe the valves have a single 'wing' that indicated water flow direction... when the wing points to the hose that goes between the red and blue lines this indicates the water will flow from the blue cold water line, thru the connecting line to the red hot water line. To use the heater, first flush any antifreeze from the system [assuming you are using antifreeze]. Once the water is running clear, move both valves in line with the red and blue pipes. This will send water into the heater... it will take a while to fill. Once it is filled, and you are not getting air out of the faucet, turn on the propane and turn on the heater switch... if the heater does not light the first time turn off the switch and light the stove top. This will help purge the air from the line if the trailer has been sitting for a while. Turn off the stove top and turn on the heater switch again... if you stand outside by the heater you will hear some clicking and eventually you should hear the 'woosh' of the heater starting.
Also, there is an 'anode' rod that need to be checked yearly. This rod will be eaten away by the acids and minerals in the water. This helps protect the interior of the water heater tank and need replacing when it is eaten away. Although my camper is still pretty new and the rod has not been eater away much, I'll be replacing mine yearly just to ensure I get the longest life out of the heater.
Again, sorry for the long post but since I don't know you experience, I figured I'd make this as detailed as possible from my own experience.