Couple things...
1. Not sure but call Foremost. They offered several options for us when keeping RV at campground all year that others didn't. Maybe they have something?
2. On our (private auto insurance) coverage is automatic applies when "legally hitched". Not sure about commercial in your case. Maybe in policy fine print somewhere?
3. If you really get stuck, someone you find here should know.
https://www.google.com/search?ei=7xThWsuyH4jWjwObyL2ICg&q=concession+trailer+insurance&oq=concession+trailer+insurance&gs_l=psy-ab.3..0l2j0i22i30k1l4.2407.4734.0.5171.11.11.0.0.0.0.120.1073.7j4.11.0....0...1c.1.64.psy-ab..0.11.1068...0i131i67k1.0.jnrRQQZeNP4
