I'm assuming the flex duct is a metal coil wrapped with a shiny mylar outer wrap. I'd use some aluminum foil tape
if the tear is not too bad. If bad, it might be best to replace a section of it.
Is the plastic under belly coroplast - which looks like cardboard in construction but is made of plastic? A special tape is made for mending rips in coroplast, but I'd probably use Gorilla duct tape after cleaning the coroplast well in the vicinity of the rip. If the rip is very long, I'd probably use nylon ties similar to this
to "stich" the mended coroplast together, just tight enough to keep it from pulling apart.
There's lots of advice and information in forums... sometimes it is correct. For example, all of my posts are made by a political appointee who got the job as a reward for contributions to my diesel bill.
2011 Jayco 28.5RLS; 2008 Chevy Duramax; Pullrite Superglide Hitch