We just purchased a 310 Seismic Wave from Freedom RV in Slinger, WI. Like you, we seriously considered buying from what I will call an Internet RV dealer. I believe that they understand that they will sell this unit to you and will probably never need to do another day of service. There is nothing wrong with this concept, but you need to realize that your local dealer will not be in a hurry to fix a camper that you did not buy from them. You will rightfully go to the back of the service line and all their customers come first. If you want a relationship with a local dealer, then that will come at a higher purchase price. At the price of gas and the time to drive to the dealer, will you really take your unit back to MI for service. Each road trip would easily cost you $500 in gas, wear and overnight lodging. I have purchased 6 campers in 25 years and have averaged 2-3 warranty visits to make things perfect. Are you game to do that with MI? Otherwise how much more might you pay a local dealer that may have more overhead and doesn't just deal in pure volume and minimal service? In my case, I got a price from the Internet Company and then a price from the local dealer. There was a difference and I negotiated a price that was acceptable for me to stay with the local dealer. Obviously, there is some price point that will make the distance worth it, but only you will know what that is.
In regards to your truck, your more than fine. I had a 2003 diesel Silverado that pulled a 351 Eagle Premiere, over 12,000 lbs, with no problems. So you should be fine. I just traded it in for a 2014 Silverado, and the exhaust brake is to die for. Almost no pedal braking necessary going down a mountain with the trailer. Awesome.
2015 Seismic Wave 310
2013 Chevy Silverado Crew Cab w/the marriage made in heaven, "Duramax & Allison".