You'll get a reasonable idea of what it's like to live in an RV--the limited space, the water and sewer and electrical systems, the compact kitchen, the often lumpy mattresses--but less of an idea of what it's like to drive a truck and trailer. Needless to say, the driving is similar to the extent that you can't expect car-like performance or handling and need to think further ahead for stopping, accelerating, turning, etc.
I think a week would probably be sufficient to get a reasonable general feel for RVing. I don't think it makes sense to rent for a month; at that point, it's probably close to a wash to buy a used motorhome and sell it afterwards, at least in many cases.
