I'm of the school that making things louder in a campground is to be avoided.
Here's a store-bought option:
http://www.campingworld.com/shopping/item/gen-turi-generator-exhaust-system/14041#readreviews
Most comments seem to be about fume improvement, rather than noise.
Some comments in this link below reinforce the backpressure=badness school of thought. Which is likely why the air-gap venturi-effect design is used in the above product.
http://www.rvforum.net/SMF_forum/index.php?topic=21854.0
It's focused on permanent-mount generators, though.
A couple genny points: In a previous decade, I had to work out of an mobile office trailer on-location at a video shoot locations fairly often, and fairly late in non-commercial areas. I ended up with a Honda Inverter Generator (EU-3000 IS) that is remarkably quiet, but dispenses exhaust thru a grill for dispersion, and doesn't lend itself to piping elsewhere. But it's got an effective muffler for sure.
During the research leading up to that purchase, I read many comments that the main source of noise from a standard generator (after exhaust) was the mechanical sound coming out of the engine (what with the explosions and all...) Worst offenders were Briggs 8-12 HP engines. No amount of muffler-foo is going to abate that. Placing them in a shallow dirt pit did, though. Not an optimal short-term solution.
Many, many folks ended up going with a pair of Honda (linkable) 2K inverter generators. Quiet as all get out, linkable for air-conditioner capability, or just use one if you're not using air. Most ended up leaving them in the bed of the pickup. But they're light enough to move where you're not, or place on the back of the trailer in a permanent (but vented) box. In that case, you could do an exhaust fan setup, with air in thru a grate in the bottom of the box, and power exhausted upwards. Would probably be good for fumes and stay cool too, if you did your math right.
If you really wanted them far away, you could put them in a box on the front of the truck, too. They're the size of small suitcases. Lots of choices...
Good luck.
Chip