If it only happens with slide coming in and not when going out, suspect the rocker switch wiring or switch contact point innards.
The switch has a complicated way to swap the wires from it to the motor as pos or neg so it reverses which way the motor turns the gearing.
If it works one direction but not the other then there is nothing wrong with the rest of the circuitry, but it has to be inside the switch for the IN contacts. The fuse doesn't blow right away but after the slide has been moving for a time? Maybe the short takes a minute to develop as the wire exposure warms up.
Should be a warranty item on a new trailer, but changing out the switch is not that hard. ISTR five wires.