You can get an air compressor and the appropriate attaching fittings. Set it to 40 psi and blow out those remaining lines. There should be low point drains somewhere. Often they are red and blue and sometimes under the rig near the hot water heater. They may be hidden by other features of the rig. Hopefully someone with your rig will chime in too.
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Tom
2019 Alpine 3651RL
2016 F350 CC DRW
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