I did only one epoxy fill coat on the bottom and painted it when camper when was 1st supine. I used a John Deere green paint. Very durable and easy to match color at a later date if needed. I painted my trailer with same paint.
I feared capillary seepage under the painter’s tape and was not disappointed. Switched to a new sash brush to paint bottom sides. Almost no seepage. Gotta have a steady hand – do it when you’re calm and haven’t been listening to the news. And some manner of arm support is helpful. It went fast for me. Finished the task with roller.
Once the camper is mounted onto the trailer, it’ll be harder than Chinese algebra to see capillary flow or not. If you want something really to fret about, try pondering about getting to hatch seals to seal properly and keep galley dry.
An engineer I know cuts firewood by measuring limbs with an electronic caliper, then cuts to length with an axe.