Regardless of whether you live in a brick n stick or an RV it seems that you can never get through the door fast enough to prevent pesky flies and other bugs from entering. “Hurry up get inside and close the door before every skeeter in neighborhood sneaks in!” Well sometimes they may not be using the front door at all.
I’ve been reading a lot about sewer flies and if your units plumbing doesn’t have P traps then yes its possible for bugs to enter through your plumbing coming up from the sites sewer through your discharge hose. But I would look elsewhere if you have a problem with Mosquitoes and Flies. First, if they are small flies look at the fresh fruits and veggies you might have stored on your counter. In an RV since the space is more confined than it is in a conventional dwelling they are instantly noticeable.
Another place to look where they might be entering is around the doors and windows of your unit. Are the door seals sealing tight? How about the windows? Are they shutting completely and are your screens free of holes? An often overlooked spot insects can infiltrate an RV is the gap between the windows and the screens, especially those where the glass slides past the screen and the screens can be slid open as well. There are brushes between the two, usually mounted on the screen itself to prevent insects from slipping between the two.
These sweeps should be checked to make sure that the space between the Sweep and the glass is narrow enough to deter insects from entering. If the gap is greater than an 1/8th of an inch, you probably should replace the Sweep with a new one or add a second one to the glass to double up to narrow the gap further. You can purchase this material which comes in a couple of different “brush heights” at a glass sales and service store. I have a couple windows and the entry door that can use some attention in this matter, so I will be doing that soon! Its just a matter of removing the old material, cleaning the surface and applying the new. Some used adhesive strips others slip on depending on surface being applied to.