The trick with getting Internet advice is everybody has different opinions on everything. So any advice you get here may or may not be good for you. I haven't read all of the replies, but I read a few from people saying you can't get to the west coast and back in four weeks. That is good advice if you only drive a few hundred miles a day and want to stop and see everything along the way. Then you may need three months.
But you guys are taking four adults. Can all four drive the motorhome? If two of you can drive you can leave Florida early Saturday morning and be at the Grand Canyon late Sunday evening. That's only two days out of your four weeks. Now everybody doesn't like to travel like that, but it is very doable. Back in the 70's my family had a class C motorhome and when we went on trips my dad and I would swap off driving. We would even change drivers without even stopping the motorhome. We could have gone all the way cross Country if we wanted only stopping for fuel.
So it would be very easy for you to get to the Grand Canyon in three days. Then spend the next three weeks going from the Grand Canyon up through California and hitting the National Parks and the Redwoods for three full weeks, and then take your last four days to get home.