After two days of study with Maester Aemon, what he learnt of decline of Night's Watch can be summarised as such : coming of Targaryens has rung the the death toll for Nights Watch and King Jaehaerys the Wise has hammered the final nail to its coffin .
Before Targaryens rise, Nights Watch's number has fluctuated between ten thousand and twenty thousand , due to incessant wars between Seven Kingdoms always some of the soldiers and highborn were sent to the Wall, when highborn who would have been executed were sentenced to the Wall they came with retinues of soldiers and craftsmen and monetary aid from their families. With unification of seven kingdoms wars decreased , communication increased, which lead to exchange of hostages being norm than sentencing them to the Wall, while Dragons are a great assets to any war, they didn't leave many enemy soldiers alive to send to Wall, the greatest bloodbath of Westeros ,the Dance of Dragons didn't leave enough soldiers on either side for the Wall. While Jaehaerys the Wise might have been a great king, his fifty five years reign of peace didn't do Watch any favours either, already Watch's numbers has dipped below Ten thousand with coming of Targaryens they dwindled to five thousand by the end of the Conciliators fifty five years reign , the donation of New Gift by Queen Alysanne hasn't helped Nights Watch any , already stretched thin, the Watch couldn't protect the holdfasts and villages in the Gift and they were abandoned due to Wildling raids, these villages produced many a second sons as volunteers for the Nights Watch.
All these reasons together has made Nights Watch as it is today , Donation of New Gift wasn't just a blow to Nights Watch either , that's near three million hectares of fertile land that North lost , that remains unused for past hundred and twenty years , North wasn't compensated for this forced donation by the Crown and it only contributed to the perpetration of North's slow downward spiral towards poverty and starvation, which led to a weaker North, that reduced the support North can give to the Nights Watch.
The South and the Crown won't be any help for either Nights Watch or the North. The North itself needs to invest in to maning the Wall ,repairing the castles along the Wall ,and repopulating the Gift and New Gift. Maybe new contracts between the North and the Nights Watch can be drawn up which are mutually beneficial to both. After all the Nights Watch is independent of the Realm and the Crown has no say in any legal business ventures Lord Paramounts undertake with independent parties , the Crowns only concern would be whether they are paying appropriate taxes.
But all of this is years, maybe decades work, not done in four days he has left at Nights Watch, he will report his findings to his Lord Father, try his best to convince him of his conclusions and ideas. That is all he can do. The Gods help him.