I've got a 2015 F450 which has the 4.3 gearing. But, my toy hauler weighs much less than your DRV. Personally, I'd get an F450, basically the same price as a decked out F350. I think the sticker on mine was $72,000 or $73,000. It's the King Ranch, black carpet, and I love the interior. It doesn't have the rear heated seats which I wanted but could not find and also does not have the engine block heater. These are low cost items but this truck had everything else I wanted and I could not find one with every single option.
In my opinion, when you get in to 5th wheels at those weights you need all the truck you can get. I would see no reason to get an F350 over an F450. Bigger suspension, bigger brakes, bigger wheels, commercial rated tires. I don't know all the weights on the Max Tow F350 but you do get some suspension and brake upgrades on the F450.
It seems some have had issues regarding registration on the F450's, especially in California. I had no issues in Texas. I'm sure you know this and may already have it, but you will need a non commercial Class A license with that 5th wheel and with either truck.
Here are some numbers on the F450:
GCWR: 40,400
GVWR: 14000
Rear: GAWR 9100
Front: GAWR 5940
5th wheel tow capacity of 31,200 but the Reese hitch is limited to 26,500 I believe.