Remove Cabinet Shell above water heater

[Xin Li]

I am totally a newbie in term of Airstream renovation. I am excited to get starting with my 1976 27' Overlander. I will have a lot questions, please bear with me, I know you all are awesome and ready to help. My first task is to replace water heater with a tankless water heater to test my skill and my patient to see whether I am up to task to a total renovation or not. If I can get the water heater replaced it will boost my confidence down the road. Much has been said, let's get to my question at hand.

I got all the screws out that fastened water heater in place outside. Now I need to disconnect all pipping to the water heater so that I can pull the old unit out from outside. To do that I need to remove the shell of the cabinet above the unit. There are quiet a few rivets I need to to remove. I tried using 3/16 drill bit to drill it out. It won't budge. Please help. Any advice and suggestion to get those rivets out? Thanks in advance.