Fixing leaking front window (worn exterior seal)

You might not have much access. My trailer had 1 inch of styrofoam sanchwiched between exterior and interior pieces of 1/8" luan board. If the separation is between the styrofoam and luan you could always drill a hole down into the styrofoam from above in order to get the sealant to the void. Then clamp. I'd be leery of using expandable foam as it might blow out the wall.