If you’re quite confident that your evil step mom is not in the drug business, or not trying to frame you… and if you really like the coach as you said you do, then it shouldn’t be that hard.
Can’t you rent a pick up truck from Avis or Enterprise and then go pick it up yourself? You could ask a friend to go with you, or if not, once there, you could probably offer $10 to a local kid to help you load it.
The only problem I see is that US border agents may want to give you a hard time for bringing merchandise. Anyways, I think that as a US resident, the first $400 you bring in is exempt of tariffs and taxes. Worst case scenario, if customs believe that the couch is worth more than that… then you may have to pay.
This scenario just made me think of something: on the way to Rosarito, stop at the border and ask the customs people what the procedure is.
Also, if the coach looks like an antique, the Mexican border agents may not let you take it out of their country without additional documentation.