[slowmover]

I'd try weight in the truck ahead of the rear axle. Mass dampening. The more weight in the trailer (distance behind the drive axle) the more potential for trouble. Polar moment of inertia. The truck has a lot of miles on it. Were I to attach a lever to either frame rail at the rear I would be able to obtain more movement per one rail versus the other compared to when the truck was new. Put some sort of "paint" on the trailer hitch connection points to see if "wear" is pronounced in some fashion. Reload the sand bags in truck side/side to see that can be "offset".

I realize this trailer is short so that this seems counterintuitive. But the trailer wheel is at the end of the "lever". Experiment in more than one direction to try and see relationships is the recommendation. Studies in N-V-H show some oddities like this. A quiet but rough-riding vehicle is considered "better" by most than a smoother-riding but noisy vehicle. In the same vein, actions on the truck at the steer axle are less annoying/alarming than those which cannot be "seen" as they are from the rear.

Same for tire pressure on the truck. More than what is required (load versus pressure) amplifies. Let aero resistance be more the goal than worrying over last bit of rolling resistance (which is counter to best braking/handling at any rate). Find the lowest tire pressure recommended and then test 5-lbs upward at a time. Set tire pressure according to the highest load per wheel if loading sand more to one side than another seems beneficial (CAT Scale is your friend) and use that for each axle.

There is going to be a sweet spot in re perceived best ride, braking and transition steering. FE will be fine, IMO. Delaying driver fatigue trumps the stunt of tenths of mpg per overly high tire pressure. Degadation of fine motor skills through the day is what matters for FE purposes. As fatigue climbs, attention strays.

.