Motorcycle Tire Center will treat you right. I'm not sure if they'll let you do all the work but they are by far the friendliest place in town. Ask for Mel or Kevin but anyone can help you. Call first and explain your situation / needs.
There is also BMW Motorcycles of Las Vegas for parts but they don't carry a huge stock of parts. Nice people there too but you have to pay the rate for labor and parts.
Also, we have a local Vegas website for riders:
Lots of local folks there and very friendly. You'd probably do well to introduce yourself there.
2015 1200GS Adventure