So many questions about full time living

A few thoughts:

 - How much is a cheap apartment locally? A few years of hard use and an RV value is going to go down drastically.

 - You don't say where it is. Living full time stationary in someone's back yard is against code in many areas. You don't want to commit to this and 2 months in a neighbor rats you out to the local govt.

 - What will you do with sewage? Can you connect to the house? Is the house septic system sized to handle the extra load? 

 - Does your friend have a 50amp@240v outlet that is metered? A decent size 5er will pull a lot of juice in hot weather and with 110F temps, you probably want dual aircon units. If they see big electric bills jumping by hundreds of dollars, they may get over being friendly pretty quick.

 - How do you plan to run the furnace in the winter? RVs aren't known for great insulation and if you get a cold snap, you can be going thru 30lb propane tanks every couple days. 

 - Do you have a truck capable of pulling a 5er? For a small unit, you want at least  3/4 ton and once you get above around 12000lb GVWR, 1 ton trucks start becoming preferable. If there isn't a mobile tech able to fix something, taking it into a shop gets expensive if you have to pay someone to do it. Mobile techs usually charge just to show up.

If it's not going to be moving, I would look at park models as they have more comfortable layouts.

It can certainly be done but it's often not as good of a plan as it appears originally.