Hi
Once you get the antenna outside, most of them are magnetic mount. That is a no-go with an aluminum skinned trailer. Yes, I know, that really isn't a helpful piece of information
Feeding any cable through / past the screen on a window is a bit of a compromise. You will let bugs and and bend the screen a bit. If the bugs bother you, grab some duct tape. A bit of poking will show you where the screen frames flex the most.
The alternatives are to use the antenna(s) that are on top of the vehicle already or to feed an antenna in through either the cable or satellite TV inputs. Doing any of this involves some messing with the what wire goes where in the existing setup.
Bob