Deb, as a mother of seven, grandmother of 14, and greatgrammie of one, I must say it seems like an excessive amount of sleep.
When you return home from your vacation I would definately consult with the child's pediatrician.
Some possible causes for this amount of sleeping come to my mind but I am NOT a medical person so do not feel competent to throw out "things I know about" that have been the cause of excessive sleep.
I do, however, strongly recommend that you do not put off an appointment with doc. Do not let doctor brush you off , rather insist on full check up for this even testing for certain conditions.
Twenty hours a day is simply too much sleeping for that age.
Best wishes and God bless