As far as the fuel gauge, I cannot help you out but I can give you a bit of advice on using the fuel cell as a holding cell.
I have had a 16-gallon cell on my rig for a number of years. It is plumbed into the main tank thru the filler vent via a small electric fuel pump. On the first trip out after mounting the cell my rig started running rough. When I stopped to found out what was wrong I had fuel pouring out of the charcoal canister. What was happening was the vacuum in the fuel system was pulling fuel from the cell, which was overfilling the main tank causing it to be pulled into the C.C. and being sucked into the engine causing a “rich condition”. I fixed this by installing a “ball valve” on the transfer line between the cell and the main tank.
Mount the vent as high as you can and use a fairly large hose. If not you will be pukeing fuel out of it as the fuel expands from the heat.
Here are some pictures of the cell mounted, ball valve, and vent.
1st off Thanks ! I have to ask does your cell have four nipples or just two mine has 4 Id assume one return one vent and two out to the engine and Id assume that I could tie the outs together how would I know what one is the vent and what one is the return ? If you have the ball valve there do you just leave it open all the time? Mine is brand new I bought it off a guy and he didn't have any paper work on it and never used it he thinks it's a 10 gal but it looks like a 15 to me. What is the LxWxH of yours? In the last Pic is that a vent and what did you use for the actual vent? With the sump you still used a pump right and I assume you have the pump on a toggle switch? OK sorry for so many Q I am tie 'n' things up to go on 6day wheeling trip and need to have things done by wed ! O one more how do you get gas in that with it mounted under ?