2003 Chrysler Town and Country Won't go faster.

I can not drive more than 40mph if road has a slop.
When I hit the gas paddel, the rpm goes up but not going faster. I went to the dealer and diag. There was no code for trans or engine but they noticed gas pump and filter need to replace and recommanded tune up, replacing spark plugs, change trans. Oil.
I did that and still having issue.
If engine or transmission problem, my car could covered by power tran warranty but there was no indication through the dealer computer scanning result.
what can I do? Keep fixing other parts will causing a lot of money. Anyone have idea what might having issue on my car?