How to cope with classical references?
I have not watched this drama so my interpret may not be accurate. From what is said, this line is self-mockery and ironic, but in a humorous and relaxed tone.


门墩、胖狗、肥丫头 concludes the modest values of Chinese countrymen with a ragged verse. It doesn't look like from any specific reference, at least I am not aware of. However, in modern Chinese literature about country life (especially rural reform), there were some well known lines that were in the form of 3 words in parallel, such as “篱笆、女人和狗”. A similar construct may hint/recall similar subject or value statements.
农夫山泉有点甜 is taken a famous TV commercial for 农夫山泉 (mineral water brand). This line is purely self-mockery. The subtext is "Even if I fail to struggle in the city, I become a peasant (against my own wish), at least I have sweet water to drink."
沂蒙山 is national wide known for its poverty. Even the speaker does have aspirations for what he said, the "happily ever after" tone in his words is unlikely to realize. This context confirmed that his intent of these words is purely being self-mockery and humorous.
To summarize, what he really means is "There is no other option but to struggle in the city no matter how hard it is."


When his friends heard this and laughed, it means they shared the same feeling and the subtext could be "cheers to you, Mr. Brightside!"

Again these are just guesses without having watched the drama. Having heard of it from many friends, I'll probably watch it some time...