It might be possible through connection sharing software like wcs.
The best way to do it would be a wireless bridge like the ecb-3220
Keenan Systems Wireless Store Featuring Engenius
You could plug your VOIP device into the bridge via a crossover cable then it would work indepentant of your computer.
You do not want to have the computer runing 24/7 so you can get your phone calls.
you could also use the universal wireless repeater function to repeat the signal in you RV.