Billy M,
I'm in the same situation as you. A Friend and I are looking to go to florida to do a long trip from there across the top of the gulf of mexico to New orleans, Houston, Phoenix, and then up to Vegas and back (about 40-45hrs).
Having done loads of research we have settled on an outfit called
www.sunstateaviation.com . This is because;
1)They have a fleet of new or nearly new 172SP's, so you won't run into problems re 50hr checks as they have confirmed that they can provide one with low/no hrs since the last check.
2)They only require a $200 deposit prior to arriving in the US (some companies have a habit of going bust with large sums of your cash)
3) They have no insurance restriction on flying from one state to another. This proved the biggest barrier as most companies impose all sorts of restrictions on staying away from base.
4) They generally seem the slickest/ most helpful outfit in general i.e they answer e-mails within a few hours unlike most who don't bother at all!!
Obviously all of the above advice is based on my dealings with them so far which has been back here in the UK. Once I have actually been, I'll be able to give you much more accurate feedback .
Hope this helps,
Jimbo
Sunstate Aviation