Your bus solar system is superduper. I don't think you need to change anything about it.
Do you have an inverter to make AC power? This is the easiest, most off the shelf way to charge your trike battery pack. Other ways are more nerdy.
If you go with a hub motor, I think you really need a 48 volt system. This will have much more torque than a 36 volt system, which you need to pull a trike through the dust at Burning Man. I like hub motors because they are easy to install, nearly silent, and there's no chain or tension to deal with. But they are geared to carry Chinese people fast on light bikes, so they do not have a lot of torque. You'll need to pedal to get the trike moving before you can turn on the motor and just ride.
But with a chain drive system you can gear the motor way down. So a 24 or 36 volt motor would work fine. I briefly had a trike that used a 12 volt motor with the axle touching the front wheel-- it had a ton of pull and could start me going from a dead stop, something the hub motor on my bike can't. Chain drive systems are louder and more trouble to install -- you need to make a mounting bracket that takes a ton of force (I welded one up for a friend that seemed plenty strong, but still twisted up when he used it). And tension and align the chain just right. And add sprockets so that the pedals don't rotate when you turn on the motor. But they have a ton of torque, the motor on my friends very heavy four wheeled bike could start from a dead stop.
I think 4x 10amp batteries is plenty for a hub motor system. Those will definitely last all the way to the trash fence and back, probably more than twice. And you can make them last longer if you pedal a little more... And, in that size, you can go up to 12 or 14amp lead acid batteries without paying all that much more.
With the chain drive system... Well, my friend burned through three car batteries pretty quickly. I think the problem is that because the motor could do more, he was having it do nearly everything. Starting from a dead stop takes a ton of power. So I think, if you go that route, either buy slightly larger (or more) batteries-- at least 20ah-- or have enough self control to help get the bike rolling and help the motor by pedaling when it is bogging down in soft spots.
Oh, and remember that the little AGM batteries get pretty mad if you discharge them and then leave them empty. I destroyed a set by running them flat and then leaving them alone for six months, they lost most of their capacity. When I pulled them out at the next Burn they didn't hold a charge at all, and I had to scrounge another set to use the second year.