Marauder stopped taking air

I don't think it is a problem that can be overcome with a tank - if you are getting to 3000 psi with the hand pump gauge but the fill valve does not open on the gun, then I would think you most likely have an issue with one (or both) of the gauges not being accurate. If they are close to being right, then you would have a 1000 psi differential and that would certainly open the fill valve.

I would think it most likely that the gauge on the gun has failed pointing to 2000 psi somehow, and the gun is already at 3000 psi - that would stop the fill valve from opening. Of course it could be something wrong with the pump's gauge too. Do you have another gun with a gauge on it to double check the pump?

How does the gun shoot? Like it is running low on air (possibly sounding louder than normal when shot) or like it is full?