carrying 4 mt bikes is easy:
$200 front hitch/swagman 3 bike (really 2 mt. bike)
$500 to $600 thule roof rack, 2 bigmouth bike racks.
use them 98% of time when not camping and zero expensive mods to a camper you may not own two years later. I am 8 years on thule 450 feet, 25 years on bars, 25 years stand up bike tray, and have thule racks on all three of our vehicles. They are used for canoe portage, roof pod (25 gallon gray tank/matt), bike portage, ladders, and oversized loads.
All vehicles sharing thule components and capabilities.