I'm not a farm guy, but I do know something about trailering.
As yet I haven't seen anyone comment about 4 specific issues:
1. If you have remote brakes( electric, hydraulic, pneumatic) on the trailer/wagon, then you have a much different scenario than with a single point pull hitch bar.
2. If you fill the container/containers COMPLETELY so there is no sloshing of fluid you have a different condition. Any sloshing movement changes you towing ratio by 25% or more( fluid movement slosh % has a doubling lateral gravity force).
3. Slope angle plays a part, but friction ratio and SIDE ANGLE movement is far more important. In other words wet grass or loose sand is much difference than dry dirt. Going straight vertical to the angle is MUCH DIFFERENT THAN GOING AT ANY SIDE ANGLE DIRECTION WITH A SINGLE POINT HITCH.
4 lastly, tow bx width is 44 inches so tip stability on anything with a raised height trailer is an issue for weight ratios. Very low trailer bed height is a big issue when maximizing weight distribution - especially without remote brakes.
If it were me, I 'd haul no more than 2 55 gallon barrels on their sides filled to the brim with each chocked so they can not shift or move.