In Illinois the fastest (by law) a trailer should be going is 55.  I know other states may differ, but how many states legally allow a trailer to be towed at greater than 75 MPH?  I couldn't resist <g>

On a more serious note - The truck tires:  I agree with the thought that a Van will probably experience more side forces from the wind than a Trailmanor will.  I don't claim to be an expert on this (by any stretch of the imagination) but would the side forces on the tires still be greater on the trailer (2 tires total for the trailer rather than 4 for the truck) since the forces would be against half the contact point(s)?

I'd be curious to hear from those who have put truck tires on their TMs just to see if they have had success.  In the meanwhile, I've purchase a GOOD tire gauge (as suggested in one of these threads) to keep track of the tire pressure...