Nice ladder Wardster. Something along those lines are what I'm thinking also. Just needs to be removable-secure when mounted.
The Stromberg Carlson alum. ladder and some of the others are simple, bracket options for mounting, fairly inexpensive. Light and easily 'customized'. Etrailer has a
video. Id rather 'bolt on' but likely build.
One, not too dissimilar from exterior ladders in construction don't care for the appearance, mostly though I want at least 4" wide rungs. I guess treads would be better description.
Leaning toward 1x's for rails to keep weight down. As were going from top of tank box, to capture bottom simple. Drill holes thru ply, grommets thru carpet, dowels or pins in rail ends to keep in place. Upper though its making something simple. Keep redirecting my self away from nice wood cleat. Wonder if the S/C cleats could be had separately... thinking out loud.
Following youtube videos just a bazillion ideas for building ladders.