If you struggle to use a manual tongue jack to raise your travel trailer, or even if you hate grinding away with an electric one, you've probably tried stacking blocks under the jack foot to reduce the amount of jacking. Here's a quick, inexpensive, and safer way to make up the difference in height.
Visit your local lumber yard and pick up a 4' length of 2" x 6" dimensional lumber. Cut the the piece into these lengths: One each of 12", 16", 20", and 24"--yes, you'll have a couple of inches left over. On top of the 24" piece, lay the 20" piece down flat, and line up the "back" edges. Screw or nail the two pieces together. Now on the top of the 20" piece, lay down the 16" piece and repeat the process. Essentially you are building a stair-step like platform. Each step is 1 1/2" taller, so that in the end, you'll have a set of steps that peaks at 6" above the ground.
When you park and are ready to use the tongue jack, simply slide the tallest "step" that will slide under the jack foot in place, and viola! Less jack extension required, and less loose material that could "slip" and cause problems.
You can build your steps taller as required for your situation by increasing the length of the 2 x 6 that you begin the project with. If you find yourself camping in areas where the ground is a bit "iffy," you could increase the lumber size to 2" x 8" or even larger to give you a wider footprint. If you use screws for your project, you may find it advantageous to drill pilot holes to preclude wood splitting and to make the screws go in easier. Many "Mom and Pop" hardware stores will cut your lumber for you at little or no cost.