We own a 2021 Wonder RTB AWD and are well into our second season on the unit. I'll try to answer what questions I can in order.
#1. We've camped comfortably with overnight temps down to -10c while fully winterized, with some planning. I'm in the process of adding some mods to the service bay to try and duplicate this with a fully functional water system.
#2 With judicious water management we can get 6 days of camping from our tanks. That includes 2 "navy" showers each. The macerator toilet does have a lower water use setting and we use the old rural septic rules for liquid waste.
#3 Any more than 2 people and you could get "testy" after a few rainy days. The shower door can be used to split the unit in half, creating two spaces. We've used this on occasion and it works well.
#4 When we have power we use the convection microwave, don't miss the lack of oven.
#5 The RTB vents well IMO. Two roof fans seem to move air well.
#6 We've yet to exhaust our 230 AH AGM, 400w solar setup but we cut our teeth on a single 100 AH battery 160w suitcase solar system in our previous TT so our power management is pretty good.
#7 No idea
#8 Fuel economy. For the current 11,000 km I've gotten 17.6 MPG (imperial) or 16 l/100 km. This is about 50% towing one of two trailers in the 1300# range.
#9 No suspension upgrades needed on the Ford chassis. I am contemplating a lift kit but that's due to my usage.
#10 It's quite easy to get the Wonder to the GVWR if you're not careful. We run at about 90% of legal.
#11 There's another thread on this site where I've posted some pictures of our 2 ebikes in the garage.
#12 The windows do sweat in cooler temps. We're experimenting with a small passive de-humidifier. We also have sheets of insulating reflecting material we use to cover the windows and the skylights in extremely hot or cold situations. This makes a huge difference.
#13 The insulation is adequate IMO. There are a few couples who use the WRTB year round in the US ski country but they are much younger than I.
#14 The Wonder RTB AWD is on the Ford chassis. One of the reasons we bought it.