Drain with the engine OFF, you don't want pressurized fuel spewing out the petcock.
Though the manuals say to drain often, some say after each fill-up, most owners do not do so at all. I've never heard of an issue from someone who did not drain regularly and most change the filters yearly anyway.
If you decide to do so, open the petcock and drain only about a tablespoon of fuel. If there's water, you will know and you can do another tablespoon. Never drain large amounts of fuel from the filter. If water has contaminated your filter to that degree, change the filter.
Larry, Eileen, and Finley
2004 Alpine 36FDDS
Third motor home, first Alpine, no need for another.