My aux tank holds 50 gallons. Full or empty I've never noticed a difference in ride or MPG. BUT, being able to pick and choose when/where I get fuel is 'priceless.'
Sherm & Terry w/rescues Steve (std Poodle) & Eydie (min Schnauzer)
2015 Jay Flight 27RLS (Camped: 102 nights 2015, 90 nights 2016)
2006 Ford F350 6.0 PSD, Lariat, 4WD, CC, LB, SRW, auto., Camper pkg.