If there is an outside recepticle it also should also be protected by the GFI. One of the 3 should be a GFI outlet with a test & reset button or the panel breaker should have a test button and a breaker arm.
if not, either the GFI outlet is somewhere else and you haven't found it, or someone replaced it with a regular outlet.
2 bits says it is an outlet since GFI outlets are much less expensive than a GFI breaker vs. a regular breaker and we all know how trailers are built! Even S&B houses seldom have GFI breakers, usually a GFI outlet