Suggestions for traveling South West towards San Diego

Wife and I are considering changing winter plans traveling to Florida for a look at another part of the country. If anyone has a recommendation for where, how and when to head west from PA. to warm climates of the south west...we have all winter, in a truck camper and can use military camping sites. Will not leave till Jan...so help point us in the right direction...any recommendations are welcome. CAN SEND PRIVATE EMAIL OR MESSAGE. THANKS