Welcome to the forum!
There's only one way to tell when the tanks get full. The shower pan will start to drain slow/not drain when the gray is full. The black tank should be right under your toilet, so you can look down the toilet to see that. It is always a hard thing to guess at how full they are, but the only safe way is to dump often.
If you don't have one, there's a brochure for your coach in the files section of this website:
I would suggest you get a can of roof seam sealant to seal the seams on your aluminum roof. The aluminum roofs are great, but they don't stay waterproof unless the seams are sealed. This typically should be done at least every 10 years, so if it hasn't been done at all, I would do it ASAP.
2007 T-286SR Cherry/Granola, #6236, original owner, current mileage: 9467.8 (as of 5/26/19)
1997 T-2653 Blue Denim, #5471
1979 12 1/2' MC, Beige & Avocado, #4639
Past Sunlines: '97 T-2653 #5089, '94 T-2251, '86 T-1550, '94 T-2363, '98 T-270SR