The dash , park brake control, supplies air to the park ( spring brakes ) on the rear axle to keep them released. Because the coach air system continually replaces any lost air , a slight leak should not be a problem.
Personally , I would want the part in hand during my travels, just in case.
Remember to completely drain the air tanks before replacement. Can't think that it is a difficult job, access to the back of the control may be tight, but that's about it.
EDIT: There are only about 4 common types of park brake controls in use in air brake systems. Most truck repair shops will have what you need.