Doesn't matter if it's a million dollar motor home or a $100 used pop-up, the principle is the same. Use a carpenter's square and put blocks under the wheels.
I have found the best place to find "level" is on the door frame with a carpenter's level. Hold it vertical against the frame, and use the horizontal bubble. Turn it 90 degrees and the other direction.
Of course with a motor home, you don't have the advantage of cranking the tongue up or down. But still, tires on board or blocks, or Lynx blocks... it's the only way.