Yes you can connect a wireless router to a cable modem and use it as an access point for your two laptops. Any wireless router should have instructions included in the box. You will get better through-put via the cable modem than through the (free?) wifi connection.
Wireless range extenders and relays CAN be made to work but are difficult for a novice.
This box may
work in "range extender mode" if your access point is using 802.11g.
The Belkin F5D7132 is a 802.11g Wireless Universal Range Extender/ access point (WURE) that connects to your network switch and lets you join your wireless-equipped PCs to your wired network in Access Point Mode, or increases the coverage of your existing wireless network in Range Extender Mode. Once you've done the setup, you can share data and peripherals, as well as a single Internet account among all your computers. It is based on standard 802.11g technology for fast wireless file transfers and downloads.
I don't know if the Belkin is compatible with an external antenna. The metal skin on your RV will block much of any wireless signal (any signal you now get must be entering through the windows). Putting any adapter in a window facing the source of the wifi signal will help.