I had a contractor pour me a concrete pad on the side of our house for
parking an RV. He promissed a 4" pour with rebar. Instead of doing
what he said he would he poured 3-1/2" with no rebar. Will this
suffice or do I need to have it redone?

I know you are asking if the 3 1/2" with no rebar will hold the RV,
but contracturally speaking, I'd get a new one. What you have received
is undoubtedly worth far less than what you paid for and did not get.

Cynthia, I agree with you. The only other option I am considering is
maybe getting a discount or having the contractor do other work pro
bono in place of a discount. If the 3-1/2" concrete will hold (it has
wire even though rebar was promised) a decent sized motorhome, I would
be willing to work something out with the contractor.

Not really any hard facts but looking at these two sites, there are
recommendations of 6 inches for the slab.  If only four inches is
used, you need rebar.

http://64.233.167.104/search?q=cache:GdYjx6BG02MJ:www.rv.net/forum/index.cfm/fuseaction/thread/tid/16100604/gotomsg/16171069.cfm+concrete+slab+rv+%22requirements%22&hl=en

http://www.absoluterv.com/concrete-foundation-requirements.htm

There is no question that you received unstisfactory work, hopefully
with a contract that specified the 4" with rebar, but even if you only
have a oral contract  (just your word against his), he was offering
the minimum allowable und the Uniform Building Code.

Here is the Wisconsin UBC, but "uniform" means that the state code has
been adapted to agree with a nationwide standard, so your state's code
is probably very similar.

http://www.biasew.org/WUBC-REGS-2005-R.pdf

Scroll down to age 18 for garages, and you will find that 4" and wire
mesh are required.  He will probably argue that he meant wire mesh
when he said "rebar" and might claim that the pad really is 4"
everywhere except where you have checked.

Your pad is probably going to crack.  If you live in an area with
ground frost, you will probably have more problems.  The guy may have
skimpt on the groundwork too.

If you can live with the potential problems, you could get hold of the
UBC and talk to a construction inspector, and then first argue that he
has to redo the job or you will report him.  After that, it depends on
how good you are at bargaining with someone who may have experience
with complaints.  But if you know that the law is on your side, you
can always go back to insisting on a proper job.

Personally, I agree with Cynthia.  Substandard work should not be
condoned.  Besides, one day you will probably want to sell the place,
and a cracked pad with grass in the cracks will not only detract
optically, it will suggest that other construction elements of the
house could be substandard.

Cynthia, 
You have some great comments here, especially myoarin.  The loads of a
motorhome may crack the pad, but if the soil is stable and the ground
doesn't freeze, it may sit there for years.  Some things might really
make a mess, however.  The soil preparation prior to casting the slab
is important.  The soil should be stripped of all organic material and
some attention should be given to the flow of surface water on the
site.  The concrete may "heave" due to expansive soils, very common in
North America, and this truly looks tacky. One may never get much
action from a concrete contractor for a project like this, any
replacement would be very expensive compared to the size of the
original work.  I suggest contacting the fellow, stating your
concerns, emphasize the warranty aspect of most contractor's
agreements, park the RV, watch the slab for a period of less than one
year, but through a wet or freezing cycle.  If there is a problem in
the first year, contact the fellow to discuss the situation.  Some
slabs like you described would last until we were all long gone, if on
the right soils and not subject to frost or soils heave.  Some slabs
would need to be 6 inches or greater with elaborate rebar inclusion to
resist problematic soild behavior.