Helen,

Your mechanic is wrong.  You must have tires rated for the load to be safe.

Most people seem to be focused on good looking tires, or aggressive tread.
 I am focused on a safe tire with a comfortable ride.  To get that, buy
"reinforced" tires, not truck tires.  A "reinforced" tire is a passenger
tire strengthened enough for the additional van load, but not rigid like a
truck tire.  Some vanagons came with a reinforced tire, some with a truck
tire.

Last fall I bought Continental 185R14 reinforced tires here in Phoenix at
Discount tire for about $55 each and they are quiet, ride nicely and seem
to handle well.  They even have an interesting tread design for you tread
fans.

Mike