I did a similar mod to my trailer (Apex 288BHS) so I could connect my Weber Q (modded to remove the regulator and add a quick connect).
The manifold under the camper was capped like yours. Removing the cap revealed a 3/8" flared fitting. I thought about removing the entire fitting, but it was so tight I didn't want to break anything. I just bought a hose from Menards that has a 3/8" flared female fitting and a 1/4" male fitting that I connected to a quick connect valve I got in Amazon.
Here is my thread with some details:
kchinth's Apex 288BHS Mods