I can only guess that originally the valves were remote operated, and possibly due to broken cables, the previous owner(s) chose to not repair and just attached a single valve. If you were to post the model number of your fiver, there might be someone who has the same or a similar camper who can tell you more information.
2016 Ford F-350 Lariat CC, 4x4 SRW. B&W turnover with extender. 2015 CC Silverback 29RE, fitted with Reese Goose Box. Amateur Extra Class K5TS, 2016 nights camped 16