1990 HR winterizing

As far as the water heater goes find the back side of it and you will see the lines going into it, if it has a bypass you will see a couple of valves. If it does not you can get a kit for around $25.00, easy to install. Fresh water tank probably has a valve on one end to empty.

You may have to just remove the Anode rod to drain the tank, this is common on Suburban tanks.
Many WH's of that period did not have a bypass valve so yo either need to add one, fill the WH with antifreeze or use air to just blow the system out.