My truck doesn't like hills?

My truck with 99k miles. Seems like it's slipping when going up hills. When it first started happening I would just hit the gas harder but one day I realized it was only doing it at 1200-1300 rpms when when I got the gas pedal about half an inch or more it would jump to 2k rpms. Yesterday it happened 2 times at 2200ish rpms. When going up a decent incline. I have no idea what it is or what I need to check. I'm not rich. So I need to be pointed in the right direction before checking everything. I'll only be able to check one thing a week because I just don't make much moneys. Thanks for your help if anyone can respond.