I thinking on upgrading my rims and tire size from 245/75r16 to 275/65r18 but I also tow a fifthwheel would or could I encounter some problems with that set-up.
Just make sure that both the rims and tires have adequate load ratings for towing; i.e. load range E tire etc... Also make sure they won't rub.
I just upgraded to 285/75R16 BFG AT KO, love them and have heard good things from those who tow with them. I let you know how they handle when I get the camper out.