Welcome to DDR!
first things first...you dont want to go to the dunes in the summertime. Both glamis and dumont are the the desert, with dumont being a mere 60 miles from death valley. summertime temps get into the 120's on a regular basis. the only way to do the dunes in the summer is late night, early morning, and even then, it is hot.
the only dunes in Ca that you can stay 3 days at is oceano dunes, in pismo beach. the dunes are on the ocean with temps in the 60s during the day
there are some dunes in Utah you could hit where it doesnt get too hot, like coral pink and little sahara, utah
dune season for us usually runs from early oct to late april for the desert dunes...you should try to make it out one new years or thanksgiving holiday