Homemade window seals for RV cords are unsafe.

No. Running a 30-amp RV extension cord through a window with a homemade pass-through seal is a bad idea.

The cord can get pinched or crushed by the window frame or seal, which damages insulation and creates a fire hazard. 30 amps is a lot of current—any compromise in the wire or connection heats up fast. A DIY seal also invites moisture and bugs into the cord end, which can cause corrosion or shorts. And if the seal material is flammable (foam, fabric, duct tape), you’re basically setting a trap.

If you absolutely have to go through a window, buy a proper RV cord pass-through kit with a grommet that won’t pinch the wire and is rated for outdoor use. Even better: install a shore power inlet on the side of the RV. It’s cleaner, safer, and you won’t have to worry about forgetting to close the window in the rain.

This is not where you save forty bucks.