Seeking advice on towing vehicle

That's a big jump up from a 21ft bumper pull. Doable but don't cheap out or you will be sorry.

We have a friend with a similar size Road Warrior. They have a diesel dually 1 ton (2013 I think)...and they've had the truck struggle in moderately hilly conditions.

You are definitely in diesel dually 1 ton territory at a minimum (or possibly 450/550 range) but also need to consider age. Newer trucks often have significantly higher capacities.

Looking the trailer up online:
- GVWR: 20,000lb
- Empty 16,045lb
- Hitch Weight: 3,520lb (presumably empty)

DO NOT JUST CONSIDER THE MAX TOW RATING!!!!

Items to check on the truck:
- Tow rating at least 20,000lb (don't fool yourself into thinking you will run with an empty trailer)
- CGVWR: Max total weight for truck and trailer loaded for the road - combined.
- Truck Payload: This includes the fully loaded hitch weight, hitch, people and any cargo in the truck. If there are after market add-ons like running boards, those count against you too.
Note, the hitch weight should be 20-25% of the fully loaded weight. If we assume 20,000lb, that means 4,000-5,000lb hitch weight.
- RAWR: Since the hitch, hitch weight and anything in the bed is almost all going to go onto the rear axle, you might hit the rear axle weight rating.

Again, especially with older trucks, you can't count on them having the same capabilities, so don't assume 2020 truck numbers are interchangeable with a 2015 truck. Also, make sure to check the rating stickers on the inside of the door. They may have different differentials or in some cases different axles. Make sure it's new enough that it has an integrated exhaust brake to help slow things down coming down mountains. If the truck has been modified, be careful they didn't reduce the capabilities (ie: replaced the tires with ones with a lower load rating to save some money...A 10,000lb rear axle with tires that are only rated for 8,000lb limits you to 8,000lb).

I don't know what that fiver weighs. But in general buy a 1 ton US brand pickup. The price difference is minor between the 3/4 and 1 ton. If you stay east of the Mississippi or in flatlands then you can get by with a gas engine. If you plan to go into the western mountains then you will be much happier with a diesel.
For a large fifth wheel you will want dual rear wheels for smaller ones a single rear wheel will do and is more maneuverable.

If you plan to stay on the pavement the a 2WD is OK if you want to travel on dirt roads (boondocking or exploring) then a 4WD. Tho you won't regret having a 4WD when you need one.

If you buy a new truck then get the hitch bed preparation. If you buy used you'll have to take what is offered. For hitches get a B&W or Pullrite.

Looking at the stats for this coach, IMO you are in dually territory. Diesel as well. 2015 and up for any of three.