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/rescue Eydie (min Schnauzer) & Charley (std Poodle)
SOLD:2015 Jay Flight 27RLS, GY Endurance (E), Days: 102 '15, 90 '16, 80 '17, 161 '18, 365+ '20
SOLD: 2006 Ford F350 PSD, 4WD, CC, LB, SRW, Camper pkg., 375,000mi
Full timing: Some will think you're crazy, some will be envious, just enjoy the freedom!