I have a DiamondBack tonneau cover on my modern 1-ton SRW diesel. I love it and therefor am not limited to TT type or really the size, yes there are some but I wouldn't go that heavy anyway. My TT is not a TH but does have a GVWR of 9500 lbs. I needed the extra 1-ton payload versus the payload on a 3/4-ton.
Get the 1-ton diesel long box for the added payload and engine braking. Having the DiamondBack is great as it still allows a fair amount of storage under the cover which is still out of the rain and locked/secured. The DiamondBack does have a weight limit of 1600 lbs so be aware of that with two utility ATVs. I have a Grizzly 700 and would not load two of those on it as it would possibly exceed the tonneau max weight with 2 Grizzly 700s and a rider loading/off loading. Would it be an issue? Probably not but why take the chance? I could probably get away with 2 loaded using a winch to load/off-load and making sure all heavy extra items are removed from the ATV storage box.
If you do go the route of a toy hauler, make sure and get one that separates the garage from living quarters. Common scenario would be traveling more than a day to get to your destination, camping for one night without really setting up camp. With no separate garage/living space, you will want to off load both ATVs so you can sleep without the gas/oil smell keeping you up.
With those ATVs loaded on a DiamondBack, that is not a concern. They are also pretty safe loaded on the tonneau as most people are not going to touch them when they are mounted up there.