90 gallon gray capacity is almost certainly two 45 gallon tanks. Easy to determine. Put some water in the galley tank. Put some water in the shower tank. Add food coloring to one of them. Using a clear adapter (that everyone should buy) on your sewer outlet, open one gray dump valve at a time. The food coloring should come out of one or the other.
The fresh tank capacity is a good question. I think Erika is correct that Heartland (always?) counts the capacity of the water heater. So maybe you have a 65 gallon tank and 10 in the water heater. Or maybe a 55 gallon tank with 10 in the water heater where because of quick recovery design, it functions comparably to a 20 gallon capacity. If that's how someone counted fresh tank capacity, I would say they were confused and should have spec'd it based on the actual amount of water. But sometimes people don't understand the things they write about.
If you're on level ground and the tank is level in its mounting (could have slipped), you should be able to fill it completely. If either the rig or tank is off level, you'll lose some capacity. A 65 gallon spec, less 10 for the water heater, less 5 for leveling, could easily turn into 50 gallons.
As many other do, I have an inline garden hose water meter to know how much water goes in my fresh tank. There have in the past been some posts about accuracy of these meters. Some posts claiming they can be extremely inaccurate. You didn't say how you're measuring to determine you have only 50 gallons, but perhaps there's room for examination there.
If you want a more accurate measure, fill the tank and then drain (through the drain valve) into a container of known capacity. Repeat until empty. Then allow for position of the drain, leveling, and typical tank sag that can reduce effective capacity by at least 5-10 gallons.
As far as the spec sheets saying 75 gallons, if you look at the fine print on the last page, you'll find a disclaimer that explains that things may change without notice. So it's also possible the fresh tank size changed in mid-year.
But frankly, after all is said and done, you have whatever capacity you have. If you want more fresh water capacity, you might look into portable bladders that fit into the bed of your tow vehicle.