Finding a black tank that exactly fits a older camper is hard to find. By chance did you find one and if so, where did you find it? We have many folks asking about this and the places that sell them pre-made are hard to line up.
There are a few plastic that will make a custom tank but the cost is even worse then the pre-made ones.
Is your black tank still with the camper? We have had many members repair their cracked, broken, mega blow holes in the tank situation to be a long term permanent solution.
If you still have the tank and want to try the repair, (it is not very expensive) you really do not have a lot to lose. But, a lot to gain.
If you have any pics you can post, we can help better, even if it has a 6" or larger hole in the tank.
On the lip support question, I'm not sure what your 89 Sunny had for the lip support. The newer tanks mount like this. The plates hold the tank flange to the bottom of the floor. The floor wood needs to be solid to hold the screws.
I have never had issues with that method, but we do not tow on the highway with a partial or full tank. The black and gray tanks do not have large steel supports under the tank like the fresh tank does. I would be concerned if one was to tow a "lot" (meaning you tow full more then empty) with the black or gray tank at highway speeds and bumps. Going slow in the CG to the dump station, no problem, but hitting a pot hole doing 50 mph is quite a jolt into the tank and water slosh, the tank bottom bellies out on these when they get full. And a full tank can upset the camper tongue weight creating towing stability issues pending the location of the tank.
Hope this helps
Current Sunlines: 2004 T310SR, 2004 T1950, 2004 T2475, 2007 T2499, 2004 T317SR
Prior Sunlines: 2004 T2499 - Fern Blue
2005 Ford F350 Lariat, 6.8L V10 W/ 4.10 rear axle, CC, Short Bed, SRW. Reese HP trunnion bar hitch W/ HP DC
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