I would have to agree with
@Ubiety that weight up top is not a good thing. However, this is something that with some common sense can be worked with. I have a 6500lb Suburban, it is not lifted, a few pounds on the roof rack will not hurt anything. Now, a lifted lighter truck that already has a roof rack with a RTT, I would be very careful about putting anything up top.
When building and packing your rig, try to find ways to keep the packing to a minimum, always load or install the heavy items as low in the rig as possible. If you need to put stuff on the roof, think about how you can move heavy stuff down, and lighter bulky stuff up.
As for Rotopax on the roof, I’m not a fan. The leaking alone would cause me to rethink the idea. I would see if there are other options to mount to the rear, side or even get a larger main tank or an aux tank.
I can just see a spark from your camp fire drifting up and hitting the gas leak on the roof of your truck and now you have a huge problem. Thanks, I’ll pass.