I have a motorhome on the workhorse P30 chassis. I’m not sure which return line to use as a drain. It has the remote reservoir, so there are three return lines.
I’m not real familiar at flushing the hydro boost systems so, welcome to the forum.
There is a thread in the forum about a motorhome that I believe would be just about exactly like Yours.
Might want to check out that thread and ask some questions about what to do to prevent them P30s from burning down.
It was a sad situation for sure.