My experience is that not much of anything will
make a significant difference in winds like that--
my Busses (and I assume your Vanagon, too) are
so tall and relatively light that 50 mph winds
really seem to enjoy knocking them around.  I
have tried different tires, (right now I have
Kelly-Springfields) and found that the better
ones help, but won't eliminate, the problem.

As for speed, my 68 has topped-out at 45 going
into a nasty west wind, and again, I think
that's just the nature of the beasts.

Mike Wright
Ackworth, Iowa
68 Camper; 79 Bus
wright@storm.simpson.edu