Not sure where else to go to level up since everything else seems to kill me just as fast.
I started a bunch of characters when I first played this game, so, like you, I've seen the beginning of the game a lot more than the middle or end. Here's my suggestion of order for places to level:
(optional first): Go get your first level killing hoppers until you get the tail oddity, then turn in their meat to Mordre. If you're *super* focused and have a good way to kill them without using consumables, can get to level 3 using Classic XP before the pace becomes painful.
First: Tanner's quest. Use some of the money to buy a few grenades and molotovs, maybe two each.
Second: Jack's quest. Grenade to kill the three goons. When in the GMS warehouse block, run to the gate and hide there, letting the guard kill the little rathounds. Pick off the Alphas for the XP. Use the money to buy a few more molotovs.
Third: Bret's quest. Use grenades to handle the beetles unless you have a better way. After coming back to SGS, clear the rathounds north and northwest of the GMS lower area, and also go into the Underpassages and kill the Lurkers.
Fourth: First level of GMS. If you have a good way to handle the robots, also the second.
Fifth: Walk from SGS to Junkyard. Pick on the three raiders west of the JY entrance, and kill a Siphoner in the next screen. If you like your odds, kill all six. Keep the best leather to craft a suit for Depot A.
Sixth: GMS 2 if you didn't do it earlier. Third level of GMS. Also take the three raiders by the bridge just east of GMS3.
Seventh: Junkyard. Kill the thugs in the screen north of the main square (the city won't aggro). Do the first two quests for the Scrappers, and the first quest for the Eels. Do Kohlmeier's quest. Do both of Abram's quests. Go back to GMS and finish off the raiders east of GMS 3.
Eighth: Go into the first Depot screen via the Scrappers route and kill the Junkyard Muties and dogs. Gather the mines if you have enough Traps skill. Go one screen west and cheese the doors to kill the dogs, and cheese LOS to kill turrets. Go back and do the second Eels quest.
Ninth: Finish Depot A.
Final stuff before or right after handing in drill quest: Two groups of Lunatics. Three groups of burrowers: GMS, SW of Junkyard land entrance, and west of the Lurkers by the Underpassages. Abram's third quest. Side with either Scrappers or Eels and do the showdown**.
Anyway, there's a fair bit of running back and forth if you follow that exact schedule, and I don't mean it as The One True Path To Success, but I've found that usually, that's about the way to run through that keeps the difficulty curve under control. So if there's any of that you haven't done yet and you're looking for places to level, consider those ideas.
**should add that there's a benefit to waiting to do the showdown until after you've turned in the drill part, so you may want to do that and then come back to JY to finish everything off, before going out and exploring the next part of the game.