While several commercial products are available to address this need, I have yet to see one that tops the performance of this little home-made ramp. If you are minimally handy (and all RV'ers should at least be minimally handy) you can construct this simple device to ensure a nice level set up at the campsite.
1 - 10 foot, untreated (I'll explain later) 2" x 10"
12 - 1-1/2" Dry Wall Screws
Good Quality Waterproof Paint
Cut the first piece to 3 1/2 feet long, with a 45 degree bevel at one end. Cut the second piece to 2 1/2 feet long, again with a 45 degree bevel at one end and cut the third piece to 1 1/2 feet long and bevel one end to 45 degrees.
Stack the 2 1/2 foot piece on top of the 3 1/2 foot base, line up the sides and the butt ends and attach using 8 drywall screws spaced two across and evenly distributed (make sure the screws are countersunk into the piece to avoid tire damage). Stack the 1 1/2 foot top ramp at the top of the stack, line up the sides and the butt ends and attach with 4 drywall screws, again, countersunk. Sand thoroughly to remove any splintered surfaces and protect with a good quality waterproof paint.
Properly constructed and with reasonable care, this ramp will last a lifetime.
Note: I do not recommend using treated lumber to build your ramp. Treated lumber is soaked in arsenic or other hazardous preservatives to slow decay and/or discourage infestation by termites and carpenter ants. These preservatives are poisonous and are not safe for use around small children or pets. Also, treated lumber tends to be heavier than untreated stocks and does not provide a good surface for the adherance of paint. Also, many RV'ers build these ramps out of 2" x 6" stock. While this works OK and is a bit lighter, the 10" stock provides a more stable base and allows for support across the entire width of the tire. Refer to your owner's manual, but most manufacturers recommend that the entire width of the tire be supported when leveling to avoid tread separation or sidewall damage.
To use your ramps, position the unit at the optimal location and, while still connected to the tow vehicle, check the East/West level by placing a torpedo level across the floor at the entry door. After determining which side needs to be raised, move the unit forward or back far enough to accommodate the ramp. Place the ramp behind or in front of the wheel to be raised and roll the unit up onto the first platform. Re-check the level and, if needed, move to the second or third level.
I often get the question "what if I need to go higher than the top level?" The top of the ramp is 4 1/2" high. If this doesn't get you pretty close to level, Bucky, you don't need a taller ramp ... you need a better campsite!!