I've dealt with this. You could have a leaking water jacket/tank (it's made of copper and brass) and in that case you could try to have the crack(s) brazed. ITR told me such leaking will, unfortunately, usually reappear. However, if you remove the access cover you'll find several internal hoses routing the antifreeze through the unit and those could be leaking or have clamps that could be loose or compromised. On mine, some of them are hard to get to, as the clamps were installed facing the rear of the unit.
2003 Country Coach Allure First Avenue 36; 2012 Honda CR-V