I have no mechanical skills but have been doing this for the last 3-4 years of owning the trailer (2012 27FB). At an RV service center this costs about $75. Doing it myself takes about 15 minutes and costs about $4 in RV antifreeze and a one time expense of about $20 for the hose that attaches to the pump do draw in the antifreeze.
Attached are 2 pics of the guide AS posted years ago (Dave Schuman of AS service, now retired) and it works for me. Would love to see VT Wanderer’s procedure too - and there’s videos and a bunch of resources on the web. I’d be thrilled to help anyone in the lower New England area who thinks they need some help. If I can do it - believe me - you can too!
Good luck!