[C&P RV]

If you had the exterior compartments open they can get in through the return air vent under the steps. There are other areas that are "open" as well. Look under the sinks where the water lines run. You can seal those areas with a rag. Last but less likely is the heat vents themselves. It's also possible that while the unit was on the lot flies got in when the door was left open and laid eggs that hatched with the warmer weather. You may just have to "bomb" the inside to get rid of them unfortunately. Good luck!