Hello @mrgrayaz,
Did you see the link in the post to the article talking about the “Taoglas Storm MA450 - 3G+4G+4GX & GPS & WiFi MIMO 5-in-1 Antenna - 700-6000MHz”? The Toaglas brand (from our experience) is amongst the best out there for using on RVs, you can source these locally though your Peplink Partner or another provider local provider.
If you want to have a separate low profile yet high gain antenna per LTE modem connection then look for something like the Taoglas Shockwave TL.10 3G+4G+4GX Magnetic Mount Antenna - 700-2700MHz
When you mount the antennas give the some separation and keep them in from the edge, the metal roof will improve the performance in some situations.
If you are going to go for separate antennas, then use the GPS antenna that comes with your MAX BR1 these work reasonably well.
Our professional advise is DO NOT use any inline boosters connected to your LTE modem (this is regardless of who makes the modems or boosters) focus on using a better antenna solution. Even with one booster per antenna connection, they will still cause more problems than they solve, not to mention most of them put way to much RF power back into the LTE modem and are likely to damage the chip set within the LTE modem (this sort of damage is not covered by any form of warranty).
Happy to Help,
Marcus 