If you really want to buy the Sentra, get the VIN from your sister and take it to a Nissan Dealer and ask them exactly what transmission is in your vehicle. Then give that info to Camping World. In their entry level cars like the Sentra, Nissan offers a manual and an automatic transmission, that's really the only option.
If the manual states only four down tow in emergency situations and doesn't mention towing behind a RV, most likely it can't be towed behind a RV. The manual will most likely not mention it can be towed with modifications. Plus it probably isn't worth putting the money into a 17 year old car to make it work. You can probably find a similar size Saturn that is newer and four down towable for about the same amount of money you'd spend on the Sentra + mods to make it towable.
-Michael
