Hmmm. I've towed with my '08 Highlander for over 140,000 miles (work and play). It is a really good tow vehicle.
That said, I've never towed anything longer than 17'. The Viking is 21'8". And my previous TT was 7'W x 99" H, whereas the Viking is 7'4"W x 116"H... considerably larger frontal area. Frontal area is a bigger concern than a little more or less weight, because it takes a lot of grunt to push air at highway speed.
Now here's the big thing that concerns me about the Viking: 406 lb dry hitch weight. My trailers have not been over 425 lb actual hitch weight. Once you add LP tanks, battery, and gear, I would bet that you will be over the 500 lb limit of your HL. And that's before you add water; you will want to see where your fresh water tank and hot water heater are located, and think about the added weight of any water you will carry. Also, remember that the weight of any WD hitch you use is also weight on the hitch, and with anything over 20' a WD-anti sway hitch is a really good idea (the Andersen No-Sway is about the lightest one out there).
You might give thought to something like the KZ Sportsmen Classic 19BH, as it would not be as high and would give you another 50 or 60 lb of hitch weight to play with.
