Since you are new to RV'ing or new to MH's with gen's here are some pics of what the Genturi looks like when you purchase it. :C
IMHO making one yourself will not provide you the safety features that the Genturi does.
1. Special Pipe material that is cool to the touch. So now one passing your RV will get burned and more importantly will not burn the side of your RV!
2. Patented air intake where it attaches to your gen to 'prevent' back pressure that can damage your gen.
Happy Camping. Jump right in and buy that Class C with the gen you won't regret it! :B