Car wash soap on fibreglass cladded trailer?

I see no reason why you couldn't, unless the bottle has a warning not to. I use turtle wax, zip wax car wash soap on my aluminum sided rig, no issues. As said, just like car washing, don't let the soap dry on.

I've found Costco the best source of car wash soap.

I been using car wash soap on these type of finishes for years. Just be sure to rinse off all of the soap or it could leave a residue.