Katrina, you might want to give
Escape Trailer Industries a call. They work on all brands of
fiberglass trailers and actually made the
Trillium for a year or so under contract. They should be able to help you or point you in the right direction for a water tank supplier.
You should be able to find the water fill door at just about any RV shop - a number of them out in Langley or there is a big parts department at Fraserway RV on Annacis Island also a small shop in North Vancouver called North Shore RV Center who will order in what he doesnt have. Here is an
RV PARTS CATALOGUE which might help you determine what you actually need and who makes it.