I built the holding tanks for my trailes from likght plywood crovered ithe inside with a layer of 3/4oz matt and one layer of 6 oz cloth also taped around the joints with abother layer of 3/4 oz mattcut into 43in strips and a layer of fiberglass cloth tape I cut the lid to size and glassid the underside the same as the box I painted all inside aeras with epoxy paint and cut narrow stripe of matt wetted them out and mushed the lis down on the box and let cure. I than rounded all the ourtside joints and crovered ahe intiee box with 6oz cloth. if the tank is being used in the inside of the trailer i would only tape the seams. If it is long fresh water tank i would put babbles in it to prevent water fram sloshong sbout when the tank is partly empty I usted to build water and holding tanks this way for boats and never had a provlem with them
By all means use EPOXY resin not the polyester resin sold at home depot lowes walmart and auto p;arts stores it is unsatisfactory.
Jim Smith
165 Tradewind (The Silver Abaltross)
1992 Dodge Cummins Diesel
Ps if you go this route I can put you in toutch with resonable priced fiberglass products.