1. Acceptable acceleration: All else being equal, the heavier the load, the longer it will take to go from 0 to 60. At some point, the amount of time it takes becomes unacceptable for safe departures at stop signs, as well as interstate on ramps.
2. As you mention, hill climbing. The heavier the load, the more it takes to keep it going while climbing a hill. Eventually, with a heavy enough load, the vehicle wouldn't be able to make any progress at all and would come to a stop. (That would be an extreme case.)
3. Structural integrity. The motive force is applied at the back wheels. Between the back wheels and the TOAD are various frame members and the hitch. During acceleration, those frame members will have forces exerted on them by the wheels pulling and the TOAD resisting the change in velocity. Those forces could cause the frame to flex or break if the load is significantly heavier than the frame was designed to handle. When stopping the same is true, but in reverse. The TOAD will push on those frame members, even if it has brakes of its own. (Think: panic stop)
4. Transmission (and differential) heat and wear. The transmission transfers the energy produced by the engine to the back wheels through gears, etc... Friction goes up as the weight to be moved increases. (Think of two gears meshing. With more resistance to the motion, the gears would press harder on each other, and scrape harder as they pull apart during rotation, most especially while accelerating or climbing hills. This happens A LOT as the transmission operates, and generates heat from friction. There's also heat from bearings, which increases with engine speed, and probably other internal sources.) Transmissions are designed to handle a certain load, and to expel a certain amount of generated heat. Put more load than that on them, and they could fail mechanically, or (more likely) overheat.
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2021 Thor Four Winds 26B on Chevy 4500
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