Al is right, the Fresh Water tank is filled in a different place. On your 1550, the fill should be a locking door (about 4" x 4" square) just to the rear of the entry door. Your Fresh Water holding tank should be located under the kitchen cabinet below the stove and sink.
Once you unlock the door, turn the plastic cap about a quarter turn and pull it off. You can try to stick a water hose in the hole, but stay near it when it's filling because it can easily fall out onto the ground. Also, since you just bought the trailer, you may want to look inside the cabinet for a smaller hose next to the fill hose. The only purpose that hose serves is to vent, but in case your tank gets really full, water will come out of that. You need to make sure that hose is connected to the end on that exterior fill door (the door has a small screen over the hole, which is about a 1/2" diameter) I had a case where the hose fell off the neck and could have leaked all over inside. Fortunately, I was replacing the exterior fill, so I caught it before I put any water in there.
BTW, the tank is not very big at all, so it will fill very quickly.
2007 T-286SR Cherry/Granola, #6236, original owner, current mileage: 9473.8 (as of 6/18/21)
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