What side of 78 will you be on, East or West? Do you want inside or outside storage? This place is new. http://www.southwyliestorage.com/ There's outside storage at Country Club road and Parker. Use Google maps and search storage and there's quite a few others.
Party Barn RV & Boat Storage. It's behind Wylie East High School on Stone Road. Main office is on 544 across from Wylie High School. They have open, covered, and enclosed spaces. Covered has electricity. I have been there 13 years with no issues.