I can't speak to your 5ver for placement, but, I put mine on the street side of the trailer so that I could watch it and
where I'm going at the same time without a lot of head motion.
You won't need to place another one on the side if you get this model of level...it has another level built into it to indicate front to back leveling. The one I have is this one:
Jumbo Level with Mounting Bracket - Item - Camping World
You can see the additional level protruding from the front. As you said, using carpenter's level(s), get your rig level in both front to back and side to side. With the Jumbo Level, mount it so that it reads level side to side and then adjust the front to back level with the set screw so that it indicates level and you're good to go.
Don & Bronwyn + 1 Cat; J-Lo
2014 Thor Tuscany 40RX (AKA, "The Cat House")
2015 Jeep Cherokee Trailhawk Towed