Hi garmp,
You are looking at 6.5 hours of driving time. Don't forget, you will loose an hour due to the time zone. Consider staying the night prior in a motel near the factory so you can get there when they open. This will relieve a lot of stress so you can give Phoenix your full attention. If you own a smart phone, consider having your spouse video record the walk through while you concentrate on the conversation. Just make sure you saved-off then deleted all your old videos and pics so you have enough phone memory.
As far as your car rental is concerned, many rental companies will pick up your rental. You don't necessarily have to drop it off at their nearest office. I would think it is common practice there in Elkhart with people like you doing that on a regular basis. Ask your local car rental company if they can pick up the car at your destination. If you are not getting the answer you want, then ask Earl Robbins there at Phoenix. He might offer some good advise.
It sounds like you already have a collection of camping equipment. I like your idea of bringing along all you need in your rental for a week long RV trip. Hint: leave the tent and air mattress home

Bring a little electric space heater too, just in case you have an issue with the furnace. When our PC was 3 months old, I learned our furnace didn't work at a bad time on our first big trip camping at high altitude Brainard Lake, CO at 10,350 feet in subfreezing temps in September. I got lucky, quickly finding a furnace kill switch outside behind the furnace panel. I switched it on and we had heat. It's that kind of thing you want a backup space heater for.
I also like your idea of staying at a local campground for the first night to make all your mistakes, and maybe find a few of Phoenix's too. Many people do that, returning the next morning to get matters resolved, or learn about operator error. So DON'T plan on getting there on a Friday for the next day Saturday they are closed. I would play it safe and get to Phoenix on a Monday or Tuesday morning. Doing so gives Phoenix more consecutive work days to assure you won't spend a weekend there.
It's going to be a long winter for you. Hang in there. Your day will come.
Ron Dittmer