When Rockstar released the 1.06 patch for GTA 5 on the Xbox 360 and PS3 fans were not happy as it brought changes to the game as Rockstar pointed out that GTA $ and RP rewards would be based on the time they took to complete. People complained about decreases in payment and more, so how can you get that all important XP in GTA 5 now?
One of the ways to get XP is with survival. Of course this will only work if you are with people who happen to know what they are doing. However while you might not get too far, you can get a lot of cash along with RP from survivals. This is one of the best ways to earn XP, but it can take time.
You can take a boat and get 4 or 5 stars and then take to the open ocean and just kill anyone and anything that gets close to you.
Another way is through GTA 5 death matches. This is the Standard Deathmatch and not the Team deathmatch. However the downside is that not that many people play it. Perhaps when people realise how profitable they can be they will become more popular. Deathmatches are also a great way to get money, expect between $500 and $1000. The best maps are Pillbox Hill and Mall Rooftop.
GTA 5 missions are another way if you play the high end ones which would bring in around $10 to 12k. Coveted mission can be played when you get to level 30 and even when on easy you can get about 1400RP and 12k every five or so minutes.