We did a similar trip in August last year.  I would go down 81 and then
pickup I 40 in Tennessee.   That is the most direct route.  I-40 will take
you all the way to New Mexico.  I had trouble with the heat in western
Tennessee, Arkansas, and Oklahoma.  When I got to Texas I thought I really
needed the airconditioning to work. (I had just bought the '87 Camper for a
friend and the PO said that the AC worked.  It was cool in Long Island so I
tried to test it but I couldn't really.)  I had my 90 year old grandmom with
me and I thought that going through the desert with her without AC would not
be a good idea.  So I got the AC recharged in Amarillo (a really nice guy at
the Texaco station right off of the exit.  He was a Christian too and
basically just charged me for the R-12 which was expensive enough).  But when
I got to the desert I really didn't need the AC.  It was hot I guess but it
wasn't the damp, sweaty heat that I was used to from southern NJ or the East.
 It was dry heat and wasn't really all that unpleasant.  So I basically felt
like I had spent that money for nothing.
I would get cruise control if I were you though or bring someone to help with
the driving.  Also stay in the left lane on I-40 as the big trucks are always
trying to pass you in the right lane and it just makes a mess.
Have fun and take lots of pictures!
Ken Wilford
Van-Again
John 3:16