Tina gave you excellent advice. To add a little to that I suggest you go to Motorhome Magazine and read the Dinghy basics:
http://www.motorhomemagazine.com/Catego ... hy-Towing/Then, they have links to all the years (in pdf format) of cars that are towable all wheels down.
To give you an idea of cost - I got a cheap 6 year old car, which I then had to have fixed to tow, went with the less espensive braking system (of several) and still had to spend approximately $10 k to have a towable car (including the price of the car). In the previous 6 years not towing, I did not spend close to that on rentals, etc. BUT I didn't stay long term too many places - and when I did, I didn't mind unhooking and taking the rv out.
I'm still not convinced I like towing, although I do like having the car at some places. I'm already having to figure out where I'll leave the car on some trips I'll take. Left it at my sister's for a trip to visit high school friends where bringing the car would have been miserable for me. And I have to travel different. Have to watch gas stations for where I can easily get in and out - and not go where I can't turn around, etc.
There are plusses and minusses - most prefer having a car. Honestly, I prefer not having one - but realisitically for my longer workaming jobs have decided to tow - for now.
I've never needed to have the extra propane bottle - easy enough to fill my propane. But some do have them and swear by them.