When the new model year trucks start to arrive on dealers' lots is a good time to buy the "old" ones. There is also the factor that dealers get rebates or discounts from the factory for the number of units sold during the sales period so at the end of the year they want desperately to make the next bracket as it applies not just to what you buy but to all prior sales during that period.
Best strategy that has worked very well for me over the years is to go at the end of the model year, scan for trucks that have been sitting for several months on a lot or moved around by dealers in swaps, go during the few days of the month, talk only to a non-commission manager (fleet manager, sale manager, internet sales rep), and keep the deal clean with no dealer/factory financing and no trade-in.
I paid less for my 2011 GM diesel truck than my neighbor did for his special order gas engine equipped 2012 GM truck.
People tend to pay way too much for new trucks and SUV's and so ones coming off lease or on the used market are seldom a good value and the work trucks are to be avoided. With a private party you get to see their maintenance records and confirm the work done and get a sense of any problems. With a dealer you haven't a clue with a used vehicle.
If you need to get a loan it is best to shop around and arrange one before you go shopping. My bank was able to put together one for me in advance up to the amount I planned to spend and would be able to provide me with a check for the exact amount needed within a day as the loan was pre approved.
With zero percent financing you are leaving money on the table as this is an alternative to a straight discount on the purchase price which also affects the sales tax to be paid.