Trail sport camper

Hello again, we just got a 2006 21 ft trail-sport camper. the guy who owned it thought the floor just had a soft spot in the floor. but after l got it home and pulled the vinyl back it had alot of water damaged from the front to about 10 ft back. but no damaged to the walls or ceiling. the floor is 1/8 plywood then foam and another sheet of 1/8 plywood. the guy only used it 4 times. his wife got sick and it was parked. how do l go about replacing part of the floor without gutting the whole inside. l was told that is what l had to do.