Hi,
Do not leave the waste hose connected to the campground sewer until you actually dump. Be sure to rinse the hose well.
I'd vote for a no pest strip on a string so it could be retrieved and thrown away after about a two week cycle to kill off all the larvae.
In side the RV I keep a clear plastic fly paper on the main fluorescent light fixture. Any insects are attracted to that. When it gets too gross--I replace it. Lots of the insects on it are smaller than the weave on my screens.