Inverters are typically not included in travel trailers until you get up into really high end ones. As far as an inverter it really depends on what you plan to run from it. If you just want to charge cell phones or laptops and such you are better off putting in some dedicated 12V outlets.
Converting your 12V battery power up to 120v only to charge a 5v phone is a very inefficient process. Yes I did it for a while, but since I boondock almost exclusively I finally decided that the losses were too much. I now run several of these 12V outlets wired to my batteries and they work well. The ones I linked are a few dollars more than average. But they were the only ones I could find that did not have a constant LED light on them to keep me up at night. I just pulled power off the overhead lights and ran some 14g wires down to where the outlets were located. I have one on each side of the bed and will soon be adding a few to the kids bunk area.
LinkIf you do need an inverter try to find a pure sine wave inverter. The really cheap ones you find online are typically modified sign wave and some electronics are not happy with them.