Have a 2014 ram 2500 with a 5.7 had cam lifters and pushrods replaced at 62000 miles got truck back has no low rpm power and is only getting about 7 mpg had it back to the dealer twice they say no codes and can't figure out what is going on hopefully you have heard of the problem and can help
Does the engine run rough? You also said there are no codes and no check engine light. I have not heard or seen anything on the issues you are having but there have been complaints about the engine running bad after installation of the cam, lifters and pushrods.
It sounds like something they did on reassembly like the timing marks not set correctly, vacuum hoses in the incorrect positions, vacuum leak the list can go on. unfortunately you may have to go to another dealer or certified mechanic for a second opinion to have them check out what they did. If a repair is needed and you have to pay you can take the recept to the dealer and have them reimburse you.