Travel trailers are not houses. The walls are thinner as well as the roof and floor. As a result the R values of the insulation are less than in a house. Also you only have single pane glass on your windows so there is a lot of heat loss (transfer) through them. In addition you have 91 to 95 degree temps comming at your trailer from 6 sides. (top, bottom, walls, front and back) There are some things you can do to help keep the trailer cooler, but expecting to maintain 68 degrees is not realistic with most trailers of that size. Try to park in the shade, and keeping your blinds closed will help, but in the end if you can keep your trailer 10 to 15 degrees cooler than the outside temp thats pretty much the norm with most travel trailers in hot weather.
