Ok, thanks so much everyone for your feedback and advice. The trailer hasn't been used since March 31s but we did have our two dogs in it for two weeks prior to that. At Jekyll Island, we had them out in the fields a lot, so I can definitely see them being picked up there. When we got home, it was time to give the dogs their flea and tick preventative medicine, so we did. We haven't had a problem in the house. I was in the trailer a week or so ago taking some measurements and didn't notice any fleas at the time. I DID, however, take a piece of visqueen I had at the house on the patio and took it to the trailer a few days back... THAT could very well be the source.
If not, I could see mice getting in there as well, as it's stored in a gravel parking lot with a ton of other RVs.
Anyway, I started the fogging earlier today and will go back and check up on Thursday I think.