Just went out real quick to measure that on my 2016 Jay Flight 26BH. Mine is 21" high but the bed supports go down inside by 1-1/2". So the effective storage height under the bed would be about 19-1/2". Good luck with your project!
2016 Jay Flight 26BH Elite, Fiberglass, Thermal Pkg, Al. Rims, 15K AC, Elec. Stab. Jacks.
2001 Coleman Redwood PUP (Sold)
2012 Tundra, DC, 4x4, 4.6L V8, TRD Anti-Sway Bar
Reese Dual Cam HP WDH, Tekonsha P3 Brake Controller