I have same boat (03 RLXI 750hrs). I drain it all down and go dry with winterization.
I still have the original heater core, although the mux switch is failing (if anyone should have one, PM me). Disconnect both heater hoses from block, and blow into top hose, water comes out the bottom one. You can blow it out by mouth, or some quick, short bursts with a compressor. I then use a funnel, pour some RV antifreeze into top hose, and blow it through til pink stuff running out bottom hose. I re-attach (all) the hoses in the Spring.