Here's my take on it. Aside from the water heater or an off level refrigerator there is not much on a travel trailer that you're going to destroy by experimenting.
Use some blocks of 2x6 or 2x8 or some purchased plastic leveling blocks to level the trailer side to side. Then chock the tires front and back on both sides of the trailer (total of at least 4 blocks)
Now unhitch and level the trailer front to back.
If you have a water pressure regulator then put it on the hose bib of the house. Hook the fresh water hose to the pressure regulator. On the trailer here should be a labeled water inlet. Screw your garden hose to the this inlet and turn it on.
Now go find the outdoor shower and turn on the hot water knob. It should take about 3-5 minutes before water comes out of the shower as the heater tank needs to fill. If the water comes out of the shower fairly quickly then the water heater is bypassed and the tank is not filling. Turn off the hot water at the outside shower and turn on the cold to bleed air out of the cold piping. Turn off the cold knob.
Now go around the exterior of the trailer and find the door to the water heater. Open the door and find the lever for the relief valve (the same valve like on your water heater at home.) Lift the lever on the relief valve and make sure water comes out. If water comes pouring out the tank is full. If no water the tank is empty. Make sure the tank is full.
Now go turn on the propane tanks on the tongue. Go into the trailer and make sure all stove and oven knobs are turned off. Then turn on and light the stove burners to purge the air out of the propane system. You might have to spin a spark knob to actually light the burners. Once the burners are burning well you can turn the burners off.
Have someone stand outside the trailer by the refrigerator panel. Now have someone inside the trailer turn the refrigerator on to the gas setting. The person outside should hear the refrigerator click three times and then light with a quiet little roar. It will try three times and if no flame it will stop trying. If the happens there was probably still a bit of air in the propane lines. Have the person inside turn the refrigerator off, wait a second and then turn it back on. Once it lights it should work fine after that by shutting the flame off when the refrigerator is cold (hours on a warm refrigerator), and then relighting automatically as the refrigerator warms.
Turn the water heater on gas and go outside. You should hear the heater light and start burning. Unlike the refrigerator this is an obvious roar.
Now plug the trailer in to shore power, flip the refrigerator to Auto and the water heater to electric. Also turn off the gas switch for the water heater. It will take about 30 minutes for the electric to heat the water heater. The refrigerator you will have to wait a few hours to see if it gets cold.
Run all the water faucets in the trailer until they stop sputtering to purge the air out of the lines. Look for leaks under all the cabinets.
Be advised that the furnace will stink really badly when it is used for the first few times. Open all the windows for ventilation and turn the thermostat on kind of high. Let the furnace run for about 20 to 30 minutes. This will burn the manufacturing oils off the furnace.
Now play with everything you can find to see if it works.
Good luck and have fun with your new trailer.
.
2001 Suburban 4x4. 6.0L, 4.10 3/4 ton **** 2005 Jayco Jay Flight 27BH **** 1986 Coleman Columbia Popup