I've never seen a trike you could really describe as anything except a road tourer cum weekend plaything unless you get into Morgan type vehicles which are really cars. For overland and off road use you have three big limitations IMHO.

1. Storage space. Luggage on trikes is a problem, the only place within the triangle of wheels is on top which doesn't help. I'm sure this could be solved.

2. Wheel track. The world's roads are cut up by trucks and landcruisers. A bike rides down one track, an outfit down the same track with the chair hopping somewhere near the other. A trike always has the centre wheel in the rough stuff. The only off road trikes I've even seen were the original Honda's that were pulled from production because they had rollover issues when ridden on rutted slopes.

3. Mechanicals. Half a VW nailed to half a Harley is going to have parts available anywhere in the world those vehicles exist. What isn't readily available is the knowledge to support you with the bits that connect them together. If your VW brakes are connected to your Harley pedal with a cable designed for a 1974 Vauxhall Viva, you are going to need that knowledge available to you. Trike convertors aren't going to be set up for phone calls at 3am local time and UPSing parts unless they agreed when you started your trip.

If trikes are your thing, get modding and testing. If you are simply looking for vehicle that'll carry more stuff than a solo bike and doesn't keep falling over when it snows, get a bike you are happy with and a sidecar.

Andy