Installing a new fitting on a polyethylene black tank??

[STLRooster]

So, a long while ago, I bought a replacement Black Tank for a 71 Airstream from Vintage Trailer Supply. I have since changed my layout and I am going with a composting toilet. I want to use the black tank as a gray tank, but I need an inlet in a different position, and I am wondering how I can go about this. It looks like i have to do a spin weld only? Is there any other way? Cheaper? What works?

I plan on putting it just aft of the axles and my bathroom will be moved to the driver side of the trailer, essentially over the axles. I hope to dump my shower and bathroom sink into the top side of the black tank (this is what I envision) Any feedback is greatly appreciated.

Thanks!!

[TheGreatleys]

You could install a bulkhead fitting in the top of the tank. The tricky part would be getting the nut inside the tank to clamp the bulkhead fitting to the wall. You'd need to already have a hole larger than the nut somewhere in the tank. You don't happen to have a 3" dump valve on this tank, do you?

Spin weld is really the right way to do it.

[STLRooster]

Thanks so much! I didn't know about bulkhead fittings. The 3" drain would be directly across from it, so I could get my arm in there to install the fitting. The only issue that is making me hesitant on the spin weld is that there is no one locally who can do this, and either way I go (buying a spin weld tool, or shipping it back and forth for someone to do) it's going to be pretty pricey.