Keith,
Try the granddesignownersforum.com. There are some folks using the Anderson hitch, but I don't remember which rig they have. The newer 303 has a 2" tube that should give you more clearance (and more room for 16" tires).
When you are ready to buy, call Grand Design and they will ensure that you have the above mod.
Nice choice of rig, you'll love it.
2019 Grand Design Solitude 310GK
2019 GMC Denali 3500 D/A SRW