I'm surprised it is a fiberglass tank. But I don't know your vintage trailer at all. If it is fiberglass, I believe you could drill a hole, and "lay up" fiberglass and resin around the fitting you want to add. Kinda like working on a boat.
Airstream waste water tanks are polyethylene plastic, usually rotomolded. This material is thermoplastic, meaning it can be melted and retain its original strength. Factories can "spin weld" fittings to polyethylene tanks. Some folks have heat guns where you can "braze" a fitting to a tank with pretty good success.
And I use rubber grommets because I'm generally lazy and can't spin weld. With this component you get the right size hole saw, deburr the hole you drilled, install the grommet, and then install the fitting you want. These grommets will not work on fiberglass tanks as they are too rough on the outside. The grommet seals on the ID of the drilled hole, and the OD of the installed ABS pipe. Most of my connections were 1 1/2 ABS pipe.