Final answer:
The most crucial action to prevent trailer rollover is checking the weight distribution, as it ensures stability by keeping the center of gravity low and even. A bouncing trailer is typically a sign of it being nearly empty, since there's less weight to absorb road irregularities.
Step-by-step explanation:
When preparing to tow a trailer, the most important action to prevent rollover is checking the weight distribution of the trailer. Proper weight distribution helps maintain the stability of the vehicle and trailer combination by ensuring that the center of gravity is kept low and is centered between the axles. Unbalanced loads can lead to sway, which increases the risk of rollover. Moreover, a lower center of gravity also contributes to stability, but without even distribution of weight, the risk of rollover could still be significant.
If a trailer on the highway is bouncing up and down slowly, it's more likely that the trailer is nearly empty. A heavily loaded trailer would have less bounce due to the weight pressing down on the suspension, absorbing more of the road irregularities. An empty or lightly loaded trailer has less mass to dampen the suspension's movement, resulting in more pronounced bouncing.