I have been taught to drain the tanks at the end of every day so as to help prevent water and or oil from compressor getting into the complex valves that trucks rely on these days. Oil mixed with the condensation becomes a thick sludge and can gum valves completely over time or slow the normal operation of the valves down. Gear changes or brake systems rely on good efficient operation of many air valves specially late model trucks.
You are supposed to drain your tanks daily, especially if your truck has only two tanks, and doesn't have a wet tank.
no
Yes, you still have your primary and secondary air tank.
For fish tanks, weekly, at a minimum.
Your fuel tanks are built to last the lifetime of your vehicle. They should only need to be serviced if they are damaged in an accident, or if some contaminant has been added to the tank.
This should be done daily, as condensation and contaminated matter could be present.
there are no drain plugs on gas tanks, you will have to siphon the gas out or take the tank down to get the gas out.
drain the tank, throw it away
On a lot of the VHD and VNL series (and it should be the same on the NH/FH/FM series, there's three air tanks under the door on the left side of the vehicle.. the first one should have a cable you can pull to drain the primary (wet) tank.
This should be on the bottom of one of the two end tanks on the engine side of the tank. There are many different designs of plugs, some of the newer styles are VERY different than the older brass, use a pair of regular pliers on them, drain cock. They still work the same, just twist it and it should drain.
Not unless you are having a drain feild problem.
Under the door on the left side of the vehicle (I don't say "driver side" because Volvo does offer RHD models for parts of the world where that's the norm, and I don't know where you're posting from) is where the air tanks should be. The supply (wet) tank has a pull cord attached to it. Pull on it, and it'll drain the air out.