I have a little 2gal Senco compressor. Needless to say, the 2gal is
exhausted pretty fast, and it doesn't build very fast either.

I also have a 5gal Harbor Freight storage tank. I would like to take the
pressure gauge off the storage tank and use that opening as an inlet from
the compressor. The compressor gauge will show the pressure in the storage
tank, so I really don't need the one on the storage tank; and the compressor
won't go over 125psi, so there is no risk of over pressurizing the tank.

Yeah, I know it takes 5 minutes to pump up the storage tank, but once full I
would have a fair amount of air; well, at least enough for my needs.

Is there anything inherently wrong with this scheme? I could pull the 1/4"
inlet and connect there, but I don't see much need for the gauge.

There is no drain on the storage tank. Is that because all the moisture
winds up in the compressor tank?