John are you replacing the front tires because of age or because of unusual wera pattern on the second tread in from the edges (rivering)? If it is just age and no unusual wear then stick with what has worked for you. If it is because of the unsual wear pattern then you might want to switch to the G645s. It appears that a lot of RVers are using the G645's.
As far as Load Range F or G that is the max weight carrying capability of the tire and usuasually measn that G will also have more plys than F. Look on your DECAL inside the motorhome or in the owners manual and see what Load Range tire is required on your MH. If it states Load Range G required then do not use Load range F. If it states that Load Range F required then you can safely use Load Range G. If you switch from F to G then you need to go up online and find the Load/Inflation Tables for that tire and make sure you run the correct pressure for that tire. That DECAL inside the motohome should help you because it gives the max loading on each axle and you can use that weight to get the correct tire pressures. My recommendation is to weigh the MH and use actual weight plus 10%. Becareful on having a tire sealer tell you off the top of his head what tire pressures you should be running. Look it up on the Goodyear factory charts. When you decide which tire and everything then this forum can help you get the correct tire pressures.
The Load Range G may ride a little harder than a Load Range F.
"Gunner" USN Retired, Airdale
2004 Monaco Signature 44' Conquest. Detroit 60