I have had my RSV for a couple of years now. Most of it's life is with a sidecar mounted to it. It never comes off (triple tree modified) and as you know mileage sucks big time. I decided to sacrifice most of my trunk space for a three gal. reserve tank. Best investment I have made for my bike, besides the sidecar. I ride exclusively on the 3 gal with main tank petcock off. Without a passenger on the open freeway I get about 80 plus change miles with the tank, with passenger I can get about 70 miles per tank full. I can go almost a full week on the 3 gal before I have to switch the petcock to on.
The reserve tank is plumbed in such a way that if I have my main tank in the on position the reserve tank will keep a constant flow of fuel into the main tank and it seems like I can drive forever before the guage even starts to move. Since the tank is mounted about the main tank line it needs no fuel pump just gravity. The 3 gal tank has it's own petcock which I leave on all the time. Down side is if both tanks are full and you park on a up hill slope fuel from the main will run into the reserve tank causing fuel to out from the over flow. If you park on a down hill slope it can flow into the mail tank and it will over flow also.
My wife and I go on rides with our club and we almost never need to fill up during the rides. One of the best investments I have ever made, ok, ok, second best. Sorry mine frau.