I wouldn't rule out a leak in roof either but your trailer is 5 years old and I don't know what the life expectancy of the foam seals is, if it happens again have the foam rubber seals replaced.
Also keep in mind that, when the wind blows the rain hard against this window it will create suction from the opposite side which may be strong enough to draw water past the seal. RV windows don't seal any better than 'yesteryears' car windows.
2018 RAM 5500 Laramie CC
Sold: Riverstone Legacy 38RE, 960 Watt Solar, 6x6 Volt AGM Battery Bank, Freedom SW 3012 Inv/Charger