With our old steps, there was no way the treads were going to all be level, regardless of the distance there were spanning. I tried putting different height wood blocks at the base of the steps, which made no difference. The stairs sagged no matter what.
Here's what they looked like when we bought the camper
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This spring we bought Torklift Glowsteps, installed on a newly built platform that will also give some storage underneath by the bumper, and also a nice "porch" - top step area. Now, no issues whatsoever with the steps. Solid and straight regardless of the ground slope. And I like that top solid step outside the door. These are the 6" 5-step Glowsteps, but with the top step being part of the landing, they are pretty much similar to a 4-step.
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The wood frame has been topped with a plywood sheet since this in-construction shot
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