Durability vs. Economy

[muttmechanic]

I'm looking into buying a new to me truck in a few months and I'm trying to decide what to go with. I'm trying to decide whether it would be better to go with the durability of a diesel truck vs having to pay for the fuel or if I should just stick with a gasoline. I have 2 people (one in an F-250 and one in a chevy 2500) that have told me they are able to get 20+ mpg with these things without any major modifications other than one having a 12 in lift. I just wanna know because I figure some of you guys drive these things and I just wanna make sure that I'm not getting my leg pulled. If they don't come stock this way do you know of ways to get them to this point without putting 10k into them. I haven't really done any research on this just kind of looking for somewhat of a starting point I guess. Thanks.