OK, here is my opinion, after doing this exact thing, twice. First, take a breath, hold it, and release. Good.
When we traveled with our children, we bought a Class "C" for the over-cab sleeping area. That MH did not have slides, but we never felt cramped. Weird, I know. We found one that was not too big (29ft) knowing we would trade up when the kids were no longer traveling with us. We bought used, making sure it was in good working condition and not too much in the way of interior repair, etc.
As for "gas vs diesel", I hate that question, because there are those on both sides that swear you have to have one or the other. For us it got down to the availability of diesel near where we live (none), and because my husband does all of the motor maintenance and has zero experience with a diesel motor, we went with gas. A Ford V10 chassis. We tow a 1998 Camry, so not a small car, and have had absolutely no problems as far as weight goes, and no problem going up hills (except that time in Colorado going up a ridiculously steep incline that had no end, but that is a story for another time :-))
I am in the "do not rent" side, because we were going to do just that, until we penned out the overall cost and figured that would pay a good portion of buying one. However, do whatever you feel comfortable with. Just know this: Once you do this, you will most likely want to do it more and more. Our children have seen most of this country, and we had a blast doing it. We had enough adventures, laughter, close calls, and everything in between to last a lifetime of memories. And that is what you are doing. Making memories. I know you are feeling overwhelmed, but trust me, once you are on the road, you can leave all your worries behind, until the heater coil goes out, haha.
Please feel free to PM me if you have any questions.
Sharon
P.S. Now we have a 2008 Clsss A, with 2 slides, and a dog instead of the kids, but we still have a blast when we take off in the winter to the south where it is warmer.