'03 4 Winds 5000 21RB Chevy chassis 79k mi. what's it worth?

Well, first off it's 20 years old. Has the roof been leaking for 6 months, or 6 years? A minor leak inside can hide a lot of damage between the roof and the ceiling. Water can leak in and travel a long way before leaking inside. Are you able to tear the roof and maybe the walls apart to repair the damage yourself?
Any 20 year old RV will have water damage of some sort. If you want a project, go for it. Offer low.