Call Roadtrek Phone: 519-745-1160 Toll Free: 888-ROADTREK (762-3873) and ask them what tank was installed on you RT. Go over to Facebook's Good Old Roadtrek and ask if anyone might have a build sheet for a similiar Roadtrek model. The build sheet will tell you the manufacturer and model of the tank. Manchester tank built the one one my 2008 and they no longer supply them. Roadtrek sold much of their vintage stock to two different places, one in Ontario and one in Mississippi. Can't remember the names of either, but a search here on on the Facebook sites should find them. The place in Mississippi had a tank that would fit my 2008 and wanted over a $1200 for it. Maybe best to find someone who is parting one out.
You need to put the right one on for it to bolt up properly - and you can't go welding on them to modify them to fit!
Found the link for the Canadian company that purchased RT parts:
https://www.mobiliferv.ca/rv-parts/roadtrek/?page=1
If I find the other company I post it here. Oh, you might try "Class B RV PARTS and Accessories-Buy, Sell, Trade" on Facebook too!