It is overwhelming when trying to decide which RV to buy. Settle on a type first, then develop a priority list. Since there will be three adults I would think a bunkhouse floorplan would be better for you. Your Mom will get tired of sleeping on a sofa very quickly. Having some private space would be very important to me.
You don't mention a tow vehicle, but if you already have one then you have to make sure you are not overloaded on weight.
I wouldn't make any snap decisions since you have lots of time before you need an RV. Make a pros and cons list of your favorite models to help you decide. Then spend at least an hour inside the RV without a salesman and pretend like you are living in it. You would be surprised the things you will discover, some of which may be deal breakers for you.
Good Luck. You'll find "the one". Take your time.