Welcome to the forum.
I can't help with your shelf question, but there folks here that 'mod' their trailers and one of them may have an idea.
Have you considered the 'Command' adhesive options? I believe they offer toothbrush holders, cup holders, etc.
Sherm & Terry w/rescues Steve (std Poodle) & Eydie (min Schnauzer)
2015 Jay Flight 27RLS (Camped: 102 nights 2015, 90 nights 2016)
2006 Ford F350 6.0 PSD, Lariat, 4WD, CC, LB, SRW, auto., Camper pkg.