Something that hasn't been mentioned yet: most minivans are front wheel drive. For towing, this is not good, as a trailer will add more weight to the rear, which can cause fishtailing at high speeds and/or with more weight. This is probably why your minivan can only pull 3500 lbs. It would also suffer traction loss from the weight at the rear lifting the drive wheels up. Additionally, stopping distance will increase on any vehicle. You don't always need a big V block to get a high towing capacity; a 2010 Volvo XC70 AWD with a 3.2L non-turbo inline 6 and 6 speed auto can pull 6300 lbs. A Tesla Model X can haul 5000 lbs(yes, you read that right!
), 5200 for a Chevy S-10, and 6100-6300 for a TrailBlazer.
As for solar panels on the trailers, I think that's a great idea!
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"You know, it's amazing how many things you can take apart with just one ordinary screwdriver!" - Calvin, from Calvin and Hobbes
"There's nothing a little duct tape and surgical tubing can't accomplish." - Sheldon J. Plankton