We usta tow a motorcycle trailer behind our RT. We used an enclosed 6x10 single axle trailer with built in shelving and hangers for carrying the riding gear and other flotsam.
The trailer with a fairly light Versys 650 and travel ready weighed 2300 lbs. with a tongue weight of about 300 lbs. A Guzzi with a hack will be somewhat more. I'm assuming(hopefully) that you ride with protective gear. Helmets, jackets, pants and boots take up quite a bit of storage so along with the number of people traveling storage will be tight. This would seem to require an enclosed trailer versus a lighter open trailer.
With a class B you will really have to watch the GVWR and GCWR numbers.
The Promaster should work, power wise on paper, though I've never driven one. We were at 11,000 lbs fully kitted for traveling with our 2000 200 with the 5.7. We couldn't fly up the mountains, especially at altitude, but we got where we were going.
As far as specifically the Solis, can you live with the cassette toilet with it's tiny 4 gallon tank? You may have to carry an extra tank. Is cabin air conditioning available for pop tops?
Our 200 Versatile RT had three seats and some had four, complete with 3 point seat belts. The rear passenger seat(s) can be made into beds for kids. Tanks were 35 fresh, 28(?) grey, 10 black, 35 gas, and just over 10 actual for LP. We mostly dry camp so tank size was/is important for us. It also had a significantly larger fridge that was not on the floor. The RT worked well and fit what we wanted to do. Sold it with 222k miles, still running well. We sold it cuz we were wanting something a bit bigger for dual travelling and found an old Safari Trek that is working well for us.
2000 Roadtrek Chevy 200 Versatile(sold)
'98 Safari Trek 2480
'15 Kawasaki Versys650LT
Perfection is a fantasy, though improvement is possible(Wifey).