Don't be so hard on yourself....
There is probably only one reason that they go with the same size tanks all around..... it makes it easier for the manufacturing.
My 5th wheel has 37 gal blk, 2X 37 gal grey , and 43 fresh.
However, the fresh water tank is 37 gal.. 'but' I have been corrected in the past to include the water in the hot water tank to be included in the 'fresh water count'.
On a chance of getting too personal....
... when you say ..."the black.... which can take a week to fill"....
Lets be sure you are using "plenty" of water during your flushes.
By using lots of water (unless you are boon docking) it will help eliminate problems with plugging and help when dumping the tank.
When it's time to dump the black tank...I always fill my black tank via the rinse connection and keep it running during the 'dump cycle'.
I will close the gate valve...fill tank... and dump again....when at CG's.
Hope this helps
'07 WildCat 30 RLBS ~ West Coast Edition ~
Trail Air - Tri Glide Pin Box
2000 F350 DRW 7.3 Diesel Lariat 89k miles
Edge programmer, 4" SS exhaust turbo/back
Vancouver Island, B.C., Canada