I actually did that through my dealer. The 40D12 was already built and sitting at the factory. It was exactly how I wanted it so we did paperwork via internet and they had it on the road to the dealership within days.
The dealer ordered the G614s through a local heavy truck tire dealer so once the camper arrived, they removed the wheels and sent them to that shop. $325/tire was my price. We just agreed to have them keep the original tires in lieu of me paying the installation price. I didn't mind.
If I'm not mistaken, if you order G rated tires from the factory, they use Westlakes. But I may be wrong. Double check that. I wanted GoodYears so just having the dealer do it was much easier.
2015 F350 6.7L Lariat CC LB
2016 Vengeance Touring Edition 40D12
Former Forest River Campers:
2004 Puma 30FQSS
2010 Sabre 31QBDS
2014 Work & Play 30WRS