I guess I will chime in on this. I have put a 103 gal L tank in my pickup under my toolbox. I was looking at tanks for months and months before I put mine in. I went with a transfer tank that I found for $100. It was rusted on the outside but I used the steel brush on the grinder and then primed and painted it. I think that once I put the tank in, pump($30), inline filter($3), shutoff valve, hose, and welded nipples into my front tank I ended up paying maybe $250 for everything. I will try to get some pics of it. I have to go change the inline filter this morning.
Oh yeah and about the gravity feed thing I was reading on here. Don't even think about that. The DPS require a pump or a shutoff valve between your aux tank and your main tank. I talked with some professional installers about this. The have some electronic valves but they are $50. So what I used was a manual shutoff rightout of the tank and then inline filter then the pump. They said as long as you have a pump between the aux tank and the main that you are legal. I' try to get some pics like I said and post them on here.