Through a window is one solution. Along the rubber wipers on the slide is another.
Ideally, there is a Marinco 20 Amp entry kit that can be installed in the RV wall. http://rvpartssource.com/Marinco-150...FYdQ7AodPAcAog
When installed, it provides a safe reliable way to connect additional power to the RV without running a cord through a window or doorway. Install it, use Romex to connect it to a dedicated 120VAC outlet in the RV and only use it with a circuit protected 15 amp or 20 amp power source .