I am having the same issues with my RadWagon4. I weigh about 85 kg (185 lbs). I also have to tighten the clamp as much as physically possible, but still the seat slips and after about 30-45 min of riding it has slipped enough to be noticeable. This is a big PITA. I share the RadWagon with my spouse, who is much shorter than me, so she needs to adjust the seat, but she can't undo the clamp as it is too tight, so she basically can't use the bike. I have cleaned the extra grease off of the seat tube, but it doesn't help, and to avoid galvanic corrosion the seat tube should be lightly greased anyway. This is just one of many problems and issues that I've had with my RadWagon. If I could go back in time I would fork out the extra cash for a Tern GSD. If you ride sometimes on the weekends, then sure, the RadWagon is fine for that, but I live without a car, and for such heavy duty daily use, the quality just isn't there with the RadWagon.
Please post if you find a working solution!