The wireless router needs to be plugged into the cable modem. Your folk's computer then has to plug into the wireless router, (all of the one's I've seen also have hard-wire ports, too)...unless it has a wireless ethernet card in it. So you could, theoretically, put the wifi router somewhere else, and run cables back to the cable modem.
the main question is, "how far is the cable modem/existing computer setup from the trailer?"
I have a similar situation at my house. trailer is 50' from the house; wifi router is at the opposite end of the house...but its not a big house. Its probably 80' away horizontally, and then up 2 stories...a couple of interior walls between it and the outside...works absolutely fine inside the trailer.
You can also get larger antennas for some models of wifi routers, that help boost the signal.
'73 Safari 23'
'00 Dodge Ram 1500 4x4 QC