Sounds like the guy giving you advice doesn't know much about electricity and wiring.
Cabling has resistance along its length. The longer it is, the higher the resistance. The maximum current cable can carry is limited to that which will not overheat it, based on the resistance per foot. However, when you get into longer runs of cable, the amount of voltage dropped across the entire cable may be larger than acceptable, even though the current is well below that which would cause the cable to overheat.
Putting 10 gauge pigtails on the end of larger gauge wire does NOT wipe out your reduction of voltage drop. You have a long run of very low resistance per foot cable and a very very short run of higher resistance per foot cable at the controller end. As long as the pigtail is large enough to prevent overheating at the maximum current, it's fine. Its length is so short that its higher resistance per foot contributes very little to the overall cable resistance.
Using pigtails on larger gauge wire is specifically what Solar Boost recommends. See page 11 of the Solar Boost 2000 Manual
You don't have to match the panel holes to the trailer ribs. Keep in mind you're going to use supports. Make the supports so that they have good attachment to both the ribs and the panels.
The Olympics may be a good thing, but so is "slathering" when it comes to trailer roofs. Make sure you do it to properly route water both when it's draining off the trailer when parked, as well as when it's being blown back by the wind of the trailer underway. When you use silicone or other sealant, make sure the area it will be applied to is roughed up, and very clean.