InesR

I would consider working on hand embroidery-I love those red work quilts or even some hand applique projects. Who am I kidding - I get car sick trying to read a book.

Just thinking out loud.
Impact forces formula F = 1/2 m v(squared) / s ; F as Force, m as Mass, V as velocity (squared in this case no way to write formula correctly), s as slow down distance.  Professor Medberry would be so proud.  But to make things simple, small objects can strike you with a force 30 times their weight.  Thus a 5 pound sewing machine could strike you weighing 150 pounds.  
Backseat only endeavor - otherwise lead story nightly news/front page morning paper X-ray picture of woman with sewing machine thread pin driven into skull in crash.  It would make a great family album keepsake.  I am surprised we do not already have one from MB.
Horizontal verses vertical thread holder pin - no X-rays - curved plastic top with no sharp edges - featherweight not the machine you want - newer plastic machine covering as much internal workings as possible that will poke your eye out or worse in a crash.
300 Watt DC to AC Inverter - will need a DC to AC power inverter.  Online it says a typical sewing machine needs 120 watts.  Double that to ensure it runs - test before leaving home with car in garage not running.  See if cigarette lighter fuse burns out when test running through multilayers of fabric.
Homemade table that sits on seat and vehicle floor - to provide a stable as possible platform while driving - sewing machine will have to be attached to table so it does not become projectile that maims, injures, and/or kills front seat passenger(s).  Table and machine weight times 30 is not the killing force.
In the end, I'm thinking you sleep all day in car while hubby is driving.  Then you sew all night while hubby is sleeping getting ready for another day of driving.  Your like two ships passing in the night, no contact or fuss the entire travel time of the trip, some may call it a win win win situation.  Husband wins as no back seat driving, You win no hotel forays as he is to tired from driving all day, family and friends win with new lovely pieced quilt tops.  Everyone wins.
Cagey
 

Here is a bag I made for my daughters birthday. She is a music teacher so think she will like it. Thanks for looking. first pic is turned wrong way and don't know how to turn on post, second pic is the inside on bag.