You would be better off dropping in last minute, unplanned without a reservation, during the Formula One racing weekend or during Austin City Limits events, or during football weekends in the Fall.
There is a rumor that Still Looking has an old Motor Home that he might let you borrow if you will allow him a few sexy photos and a quickie. You could most likely park the Motor Home at the Walmart, but there would be no utility hookup for electricity or water.
Seriously, ATX is a great place to visit, but you need to plan ahead during busy weekends: UT Football Game days, SXSW, ACL, Formula One Race events.
Avoid Williamson County at all costs to include Round Rock and Georgetown because of very active LE.