I will give you some basic instructions on how to turn it on, but if it hasn't been used in a few years, it may not start this way and will need some cleaning. But if the previous owner used it recently, you shouldn't have a problem getting it going.
1) Make sure the LP gas bottle(s) are turned on
2) Make sure you have a good, charged battery hooked up on the front tongue
3) Go inside the trailer, and using a lighter or match, light the stove burner. Leave it on for 10-15 seconds just to get gas through the lines. Then shut off the burner. You don't need to do this step all the time, I just mentioned it so we'll know you have gas in the system and it should help the furnace start quicker.
4) Locate the thermostat for the furnace, which should be close to eye level and mounted on an interior wall, like a refrigerator cabinet/box or a bathroom wall.
5) Determine which lever on the thermostat is which- there are two. The one that slides from side to side rather freely will adjust the temperature, and you should see the needle move as well. The other lever will only slide/snap side to side, DO NOT PULL TOWARD YOU. That lever will just snap one way or another (doesn't move much) to act as an on/off switch. I think snapping to the right turned it on, left was off, but it should say on it. I'm going to guess the on/off switch will be on top and the temp adjustment will be on the bottom.
6) Once you slide/snap that switch to the on position, you should hear the furnace blower come on pretty quick. Within 30 seconds of the blower coming on, you should hear the burner ignite and you should feel heat pretty quick.
If you hear just the blower but no ignition, chances are something is dirty/clogged in the gas line to the furnace. If you don't have a blower noise, then you've got other issues somewhere.
Here's a very similar thermostat:
2007 T-286SR Cherry/Granola, #6236, original owner, current mileage: 9467.8 (as of 5/26/19)
1997 T-2653 Blue Denim, #5471
1979 12 1/2' MC, Beige & Avocado, #4639
Past Sunlines: '97 T-2653 #5089, '94 T-2251, '86 T-1550, '94 T-2363, '98 T-270SR