I had a Jayco originally equipped with horiz tanks, which were subsequently replaced with vertical tanks mounted horizontally.
On horiz tanks, I believe the inner tube that picks up the vapor is bent so that it is not in the liquid.
What I would do would be to have the propane filler only fill the tank about 1/2 to 2/3 by weight
so the tube wasn't in liquid when on its side. Occasionally they would over-fill, so I would have to dismount that tank and use it vertically until the level dropped enuf.
If you can't find any OPD Horizontal valves, you may have to resort to that.
Marine stores, like West, carry horizontal tanks in aluminum, but they are a bit pricey (like $150) and I dunno if they are OPD...
Pete in the RatHaus