I doubt you will find a tank ready made that will fit your trailer. Welding plastic on these tanks is not a good idea either. You can get a tank built to fit the space you have. Take out the old tank and measure it carefully, especially for fitting the toilet in properly. Do a dimensioned drawing so you have something to send to the tank manufacturer. One tank supplier is
You will likely have to take the toilet perch apart as the tank comes out from the top. Take out the valve from the bottom, take the toilet out, take the perch apart, and the tank should lift out. You have to cut the drain pipe to get the valve out so keep the pieces and take photos of what it looked like before you took it apart. You will rebuild the drain system after the tank is replaced and the pieces and photos give you an idea of what to do as you rebuild. Get a new valve to put into that new tank.
I'll repeat: measure carefully. The tank manufacturers build what you request and, if it is not right because you sent them the wrong information, you will wind up buying two tanks.