Just levelled my guys up from 2 to 5 today taking on loads of single-player campaigns. The last few I've finished though, no more exp gained when collecting loot, which makes me think I'm wasting potential exp here. Is it only the last fight in a campaign that grants exp?
Yeah, only completion of an adventure that nets you xp. If you want to grind an adventure, it's possible to just resign before the final map.