It seems that the manual slide overrides are located in various places depending upon the year and model. Ours, for example, is found outside -underneath the RV and hidden by the black coroplast underbelly covering.
It is located about a foot or two in front of the steps and about a couple of feet in. Essentially, it is in line with the outer leading edge of the slide. There is a small cutout, about 8 in sq ) or flap in the coroplast which is held in place by ZIP ties.
Once this flap is lowered, you can peek inside and see the manual override. You have to be on your back or belly to turn the shaft with a ratchet. The instructions tell you to disconnect the connect to the battery before cranking the slide by hand.
I've never had to use it but I'm sure it will be on some day when it is raining hard and the ground underneath the RV is soaking wet and muddy.
Are you able to get in touch with the previous owner and ask about the location?