ventureroyal

Hi everyone, I own a 1989 venture royal, Problem is when cold the clutch works well until about 7-10 miles into a ride then the clutch doesn't work. I replaced the fluid and the slave cylinder thinking leak by when it got hot but to no avail. One mechanic is leaning to replacing the clutch plates but it don't seem right. Has anyone had this problem or can anyone offer up some advice? Thank you