I realize this is an old thread but I wanted to add my .02¢. I currently have an '06 wing with roadsmith HT1800 and no aux fuel tank. On a couple occasions while traveling unknown roads, I ran verrrrry low on gas. To the question of the gas in the aux tank getting stale, all you have to do is use it. Where I live, my wing isn't touched from about mid November until mid March.
I use SeaFoam to stabilize the gas in the tank. That could be added to the aux tank when you fill it. It will clean the fuel system also. Someone mentioned the added weight cutting MPG. Gas weighs about 6 lbs per gallon. With a 5 gallon capacity, you're only looking at about 30 lbs for a full aux tank. I really don't think that will make a noticeable difference in MPG.
I pick up my new 2019 goldwing/roadsmith trike next week and you can bet the aux tank was top of my list.