I have a Class A but I have also used the Earthmate sitting on the dinning
room table sitting up my laptop where the nearest window was about 4 feet
away and I received a good signal.  I would think anyplace on your dash
should be acceptable, but I would try to get it near level.
Ed

I see where Bill Lederer <bill.lederer@verizon.net> contributed:

If you haven't tried just sitting the receiver on the dash at the base
of the windshield, you won't know whwther you have a problem or not.
But I remind you that radio waves are not significantly attentuated by
fibreglas.  Hint hint.

Will Sill

Bill, it somewhat depends on what kind of roof you have.  I had a Minnie
Winnie with an aluminum roof and reception was pretty bad anywhere
inside.  I finally got an external rebroadcasting antenna and mounted it
on the outside magnetically on the refrigerator dome with the
rebroadcasting antenna stuck to the windshield with a suction cup.
Worked great, and I still use it in my RV with a TV overhead in the
entertainment center.  I place the external receiving antenna on top of
the TV inside the coach and the rebroadcaster is still on the
windshield.  Works great.  It helps some because the heavy electronics
shields the overhead satellites from the GPS on the dash.  I use an
older Garmin emap and switch it easily to the toad that does not need
the extra antenna.

If you have a rubber roof or a fiberglass roof, your GPS should work
just about anywhere inside where there is little metal shielding for the
antenna.  Keep in mind that the GPS systems use a very weak radio signal
that is just about at the background noise level.  Each satellite has
its own quasi random but known signal signature. That means that the
signal can be blocked pretty easily.  Your receiver searches in the
background noise until it finds a match to the signature value.  Voila,
then it knows what time it is, and where it is relative to that
satelite.  An external antenna on the roof is always a good idea because
it gets the best reception there is.  High trees used to block my
signal, but not any more.  My fiberglass roof does not block the signal
at all, but the TV sure did.  I couldn't see any satellite that was
directly overhead.

John Andrews, Knoxville, Tennessee

Are you wanting to use it while moving down the road?  Is the cab-over
area aluminum or fiberglass skinned?  If fiberglass, you can extend
the USB cable reach with extensions and put the receiver as high as
you can (closest to the roof) and not beside any of the electronic
equipment (TV, VCR, etc). and figure out how to route the cable down
to where the computer will be used.  
Gary

My GPS works almost everywhere, even the dining room table. Usually its
just on the dash against the windshield and it works fine. The rig is
fiberglass

Traveling