Welcome!
Games can definitely be moved horizontally, either on their sides or back, but there's a caveat with that: not everything inside an arcade cabinet is bolted down like it's supposed to be, so before you tip it over it is a very wise idea to open up the cabinet and check everything.
Monitors are supposed to be bolted to the cabinet, but occasionally people will undo the bolts and forget to put them back, and as soon as you tip it over the monitor shifts and bad things happen. Also, one time I moved a game and verified the monitor was bolted in place, but the chassis (the pcb for the monitor) was loose within the monitor... it shifted when I was moving the game and I didn't notice it. When I put the game back upright and plugged it in, the chassis shorted to the monitor frame and killed the monitor.
Power supplies and transformers, usually at the bottom of the cabinet, are often very heavy and if they move around they can do a lot of damage.
PCB boards are not usually very heavy but they are fragile. I sometimes just remove them and keep them in the front seat. Option B is that they are usually attached to the side of the cabinet, and so when I tip a game on its side I try to tip it toward the pcb side, so if the boards come loose they have nowhere to fall.
Coin boxes are not usually secured at all, and depending on the design they can rattle around inside when you tip a game, and if they hit the board or the neck of the monitor, you will be unhappy. I usually remove them before moving a game.
The control panel typically opens on a hinge, and usually holds the monitor glass in place. So if the control panel is not secured, it can flop open, and may allow the glass to fall out... again you will be unhappy.
As far as getting them into a truck is concerned, if you have two people it's not too hard. Just place it 3-4 feet from the open tailgate, tip it gently back until the top is resting on the tailgate, and then get at the bottom and lift and push. A blanket or a big piece of cardboard can help the game slide in, as well as helping to protect the game (and the truck, if you're worried about that). Same technique with one person, but you have to be a bit stronger and you have to be careful that the bottom doesn't slip out as you're tipping it towards the truck... this is a bigger issue with the games that have wheels at the back.