My V6 2014 has fuel cylinder shut off too. As noted, rough in the low 30 mph range. I've pull granted low windage trailers to 8000 with no issues. Something else as noted by others is wrong with your truck.
But, if you are going to be pulling as noted for a serious amount of time above 6000' elevation, the ecoboost frod makes sense over a non forced induction motor that loses 3% of it hp per 1000' elevation you gain vs sea level.
Reality, for a single trip, you 5.3 is more than capable assuming running correctly. But it will be slower at higher elevations than a forced induction motor like that ecoboost, turbo diesel etc.
Marty