For mine you pop out the fresh water tank/closet panel and reach down to the valve it's far to the right. You could easily add a valve to the Pex drain pipe on the outside. Then just keep the water drain valve open. Inline valve simple simple. Or put a T in with a 12v water valve, not as simple but still pretty easy. You could put an access hole in, pretty simple, cover it to prevent drafts.
Come to think of it my drain line for the tank is a separate line with the drain valve. If I went electric I would want a valve that I could open and close not one that is normally open or normally closed. Bet they are our there. I keep my drain valve open during winter months when not in use. I'm goin to look for one and I'll post it if I find it.
My '91 had its valve under the lift up bed. Had to let the bed, ugh.
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