12v transfer pump. Empty/use up the main tank as normal, then use the TP to refill it from the 2nd/aux tank (which I assume is smaller than the main FW tank.) That way, you know when you empty the main tank and refill from the aux, you're getting close to being empty. "Reserve" if you will.
2015 XLR Hyperlite 30HFS5 (mods being performed regularly)
2009 Salem LA 292fkds (gone)
Nights- ('12)23 ('13)23 ('14)15 ('15)31 ('16)25
2014 Harley Davidson FLHX (XLR cargo)
2014 VW Jetta TDI (diesel sipper, camping support vehicle)
2011 Ram 2500 CC 4X4 CTD, B&W Companion (toy hauler hauler)