[Bullwinkle]

4th on flat ground, downshift if needed going uphill, and downshift on the longer downhills so you're not riding the brakes. At times you can go to 5th on mild downhills & coast to save gas.

Tandem axle trailers should have brakes on at least 1 axle, and if it has electric brakes (which it probably does), you will need a brake controller.

Also, anything with a trailer is limited to the 2 right lanes on a freeway by law.