The very best way to increase your towing capacity is to trade your RV in on one with higher towing capacity. Not only will you wind up with a rig actually designed by someone who knows what they are doing, to get that higher capacity, you'll find it cheaper especially in the long run.
To modify your current rig, you'll need a new (stronger) engine, transmission, drive train including axles and brakes, beefier suspension, and if you want your insurance company to not have an excuse to deny coverage when you need it most, you'll need to have all those mods blessed by experts in advance, too, as well as signed off on by said insurance company.