I hope you have a screw in hole on the top of the tank....makes it easier all the way round. Secure the tank. lay the plywood floor and install it. Plan for future maintenance when you do that. Have the "hole" cut above the tank. The toilet flange will now screw down thru the floor to the tank. Be sure the flange you get has a close floor hold down spacing when fully screwed in. Use some thread sealer...not glue...on the threads. You will need four toilet hold down flange bolts and nuts to holt the fixture too. I think the one on Vintage Trailer Supply web site would work....but, you can get those anywhere. You can get some infor from Steve at VTS as well. I think on my "Special Order Bubble" forum subject posting has some pictures too.
"If it can't be reduced, reused, repaired, rebuilt, refurbished, refinished, resold, recycled or composted
then it should be restricted, redesigned or removed from production."