Right now from what I have seen and heard US embassy is being lenient on giving visa. Just submit your I 134 and your bank statements, pay stubs, tax returns etc. all financial documents to prove you can sponsor their trip.

Include a letter asking your parents to visit here and nice letter to consular that you are inviting your parents for few months and they will go back for sure.

Also make sure your parents can make good tie at back home so that consular can believe they will return to Nepal after the trip. If you include all these you should be fine.

Good luck!