I have the 25XX series ADCO on my Sprinter. Great for light, heat and privacy, but the mesh size will not stop no-see-ums. It may stop most flying insects, but not crawling insects. Too loose a fit. I'm sure it can be modified, but unless you change the mesh no-see-ums will come right in.
Might be a clogged vent tube. Located on passenger side of generator at the bottom. Sleeve type black foam filter. Yes, regulator could go bad. When my generator gave me start problems it was the oil level.