7 nights in Athis Mons near Paris? Renting a House?

First off.. which airport are you flying in and out of.. because Athis Mons is near Orly airport..not Charles de Gaulle.. so check where you are actually flying into.. It is not at all convenient for CDG, but close to Orly.
It is served by the RER C line.. but I have a feeling 20 may be a tad optimistic.. but not horribly so..

Also how far is HOUSE from RER station in Athis Mons.. do you want a 20 minute walk from station to house in dark..
Personally I do see the attraction of having the car.. but I also would prefer to be in city so I can just walk out my door and "be there".. and to me 7 days is not a long visit.. so its not like you could do that many daytrips AND see much of Paris.. 7 nights in only 6 full days after all.

Ultimately I would ask more questions.. and it does come down to what you enjoy..

Have you looked up the town to see what its like.. ( they are not all scenic..my relative moved out to a new suburb.. well. it was not some adorable village.. it was more like modern apartment buidlings )

Bear in mind that Athis Mons is on the RER C and not the Metro like it sounds like the other place was. You will need to get point to point tickets to get into Paris, which is not a big deal but something for you to know. Once you get on the RER C, it will be about 30 minutes to the center of Paris, which IMO is not too bad.

I think I've decided to take this.. there are always alternatives, but I feel like this will be close enough to transportation and it fits our timing and desire to purchase lodging using some existing timeshare points rather than paying cash. I can't find their exact address in advance... but they say convenient to transportation and say 20 minutes to town and 10 to airport.. so I can't imagine it's super far walk to stations or at least to a bus stop. We will fly into Orly from Rome.. so the arrival will at least be convenient.. If we have to depart from the other airport I imagine we'll find a way to get there for the one time trip.

Thanks for the heads up on the RER vs. Metro. I'll look into a pass that covers both.

Look into the Navigo Decouverte Pass.. It covers a week of travel BUT it set to run from Monday thru Sunday ( so if arriving on a wed-thurs etc.. may not worth getting really )

It gives you unlimited travel all over paris and out through ZONE 5.. however.. you had better check with a local if your station is actually within zone five..

I strongly recommend you ask on tripadvisor.com paris forum.. I know locals there can be a lot more helpful .. I have been travelling to Paris for decades.. but when needing to visit an elderly relative just outside Paris I could not figure it out.. because relatives gave me wrong name of closest station.. a local on the forum knew exactly what I needed to do to get to them.

I cannot imagine committing to a place based on 'their saying' it is convenient to transportation. You need to know WHERE it is before committing. They could give you the closest cross street if they don't want to divulge the exact address for security reasons; failure to be willing to do that suggests they are hiding something. I would go to that intersection on google earth and look around. And no way it is 20 minutes -- you are probably looking at closer to 45 with walking to the station etc.

I rent places all the time (have done 95) and landlords have always been willing to give nearest cross street or rough location of the place. With that information you can look for nearness of transport, grocery stores, bakeries etc. Again, a landlord who won't give you the nearest cross streets or similar location is trying hide something from you.

And IMHO given how very easy it is to do spectacular day trips from Paris by train for the 21 Euro a week your Navigo Decouverte costs, it would make much more sense to stay in Paris at an AirBNB and do day trips from there than to rent a car which is incredibly costly. Take a look at some trips we have done by train: https://janettravels.wordpress.com/category/day-trips-from-paris/

with a place in Paris for a base you would also be in charming Paris instead of some suburb.

But especially don't put down money without knowing where your RER station is related to your lodging and what the area looks like, what amenities it has etc.

When it says "10 minutes from airport", how often is that bus? Every 10 minutes is different to once an hour or every couple of hours. Does it run 7 days a week? Does it go in the evenings.

What Nigel said. Any time someone writes 'steps from X' or '10 minutes from Y' I know they are blowing smoke. Rome is 'steps from Paris'. A lot of steps.