We discussed some of the issues on the Yahoo site, but we can take up the particulars here.
New or used:
One of those would be fine. Actually, not knowing your finances, this is not something any of us can answer. If money is no object, one would certainly say, buy new and pick out all the options, fabrics, etc. But there is a lot to be said for a clean, two or three year old unit that has had the "bugs" worked out of it.
Depends on the ratio of travel time to stationary time. It takes no genius to figure out that longer and wider is more roomy than short and narrow. But short and narrow is easier to pull. Since all the late models, except the International, are wide bodies, and people are recommending a newer model (rightly so, I think), the issue is length. A 34' will require a major truck. 25' is, for most people, the minimum size for fulltiming. In general, the extra length of an Airstream gets you mostly storage space - look at the floorplans on the Airstream site.
You might be a candidate for the now-discontinued 27' Safari.
What kind of vehicle to use to pull:
Many of us are partial to diesels from GM, Ford, or Dodge. The extra low end torque makes towing a lot easier. There are also good gas engine options available. Do a search on this site for tow vehicles and you will find a lot of information.
Do I need to tow a small vehicle:
No, unless you buy a motorhome.
Do campgrounds allow pets to run:
How do I learn to responsibly drive one:
There are schools. There may be ads in Trailer Life. Failing that, attend a rally and ask lots of questions. Learn about hitches. Learn about bow waves. Learn about turning radii (plural of "radius" - didn't think I would know that, did you?).
I really like the shower in my 25' Sovereign
, but I believe that all the longer trailers have even larger showers.
Easy access to stored items:
One of the real strong points in an Airstream.
Good repair service:
Within the context of RV life, there is probably none better than Airstream
The problem here is that EVERY maker turns out a significant number of trucks that are no end of trouble to the owners. You can get lots of recommendations for every brand, but the particular one you purchase may be a loser. All you can do is go with the averages. Stay with GM, Ford, or Dodge. The ability to choose from full size van, Suburban, Expedition, Excursion, or Pickup truck will depend on the size and wieght of your trailer. Getting enough truck and gearing for your trailer will be more important than the brand.
What does the sign say? You can have it cheap, quick, and done right. Choose any two. Something like that. I bought a 85 Soveign 25', and a '94 Chevy diesel for a combined price of less than $15,000. But then I am handy with tools. The new equivalents would be, oh, maybe $75,000?
Keep asking questions until you get the answers you need.