The easiest way to build a ramp is with lumber. A trailer tire can rest on a surface as wide as the tire and about 8 inches long, so you need to know the width of your trailer's tires. Build a ramp by stacking increasingly shorter pieces of lumber four or five layers high. When you are at a campsite that is uneven, place your homemade ramp behind the tire on the lower side and back up step-by-step until you have evened out the relative height of the each side. This will allow your trailer to be set up more easily and make moving about inside more comfortable.
When you buy lumber, the size referenced in the name is not actual size. For instance, a 2-inch-by-8-inch piece is actually 1-1/2 inches by 7-1/4 inches when you measure it with a tape measure. You will need to match the actual size of the lumber with your tire, since a tire that rests entirely on a board is more stable than one that is only partly on a board.
Next, you want to figure whether you want a three-, four- or five-step board. For a four-step board, the lowest step needs to be 4 inches by 8 inches, the next 3 inches by 8 inches, then 2 inches by 8 inches and the final step will be 8 inches. Adding the total inches together lets you know the length of lumber you want to buy, with one addition. You'll want the end to be 1-1/2 inches times the number of steps you use, plus an additional 1 inch to 1-1/2 inches as a stop for the end. If you decide to angle the step ends, a 45-degree cut would make sense and you could add an inch to each step to account for the angle cuts.
Gluing and screwing the steps together would create a stable tool. The screws could go 9 or 10 inches in from the top edge of each step. Drilling a hole to start the screw makes it easier to insert the screw, as does using some lubricant, like a bar of soap. Flathead wood screws would be preferred and should be 2-3/4-inches long to keep them from going through the lower step while allowing enough connection to make a strong bond.
