Check the oil level. If it's marginal then when it's running at a low engine speed it's fine. But load it and as the engine revs up the oil gets slung around the crankcase effectively lowering the level in the sump which could be tripping the oil level sensor and shutting the engine off.
If you're overloading it then it should lug down and kick the breaker. But it just shutting down when loaded tells me a safety alert is kicking in, oil level is the easiest to check.
And it doesn't have to be "Low" to trip the sensor. My Honda can be halfway down the crosshatch on the stick and trip the level alert.
Now-2014 Sierra 346RETS 5er BUB
Then-2002 Keystone Springdale 286RLDS TT
Nights camped in 2014-28, 2015-127, 2016-10