In my MH, the antifreeze is sucked using the water tank pump and into the lines, having blown all the water first using a compressor first to blow all the water out. The compressor line connected with a suitable adaptor at the city water intake.
The water heater draining plug is likely screw type. Try getting it loose using a vice grip, but don't apply too much pressure on the plug.
1999 GBM CruiseMaster
Toad: 2017 Toyota Corolla SE