Hello and welcome.
Just about everything you want to know about an Axis/Vegas can be found in Ed's excellent crowd sourced manual (if you haven't already) download and read it:
http://www.thorforums.com/forums/f10...nual-4961.html
Now to your specific questions:
- Leveling system: yes you can get by without one.
- 15k A/C keep it cool enough: depends on where you travel, In the midwest/northeast yes (we make do with our 13.5k) in the Southwest, maybe not. That front window really lets the heat in--a good way to mitigate that is with a Magneshade or Sunguard cover.
- Sewer: Haven't had any issues. It really isn't "uphill" as there is a hole in the bottom of the compartment--you don't run the sewer hose out the side hatch. (If the unit you're looking at doesn't have the hole/removable access panel they forgot to put one in and your dealer should fix that.) Also note that the sewer output rotates (usually we drive with it slightly up, then open the hatch and rotate it down when setup).
- Suspension: First thing you should do is get an alignment (since you're looking at a used one the prior owner may have done this but it wouldn't hurt to get it checked). Everything else after that is up to you based on how you like the way it handles (all we've done is alignment + front sumo springs)
- Spare: Some people have used a hitch mounted spare, others have fit one between the framerails underneath the back