Have you used the outside shower? (#1 cause of this syndrom)
It is possible something has changed.. Atwood water heaters come in a few different ages.. Modern ones use one T-Stat and one ECO.. it may be the T-Stat has wore out and is cutting you off sooner, or some other related change.
NOTE: you can get T-States in other temps (120,130,140 up to I think 180) however goign higher than 120 is not recommended.
Another posibility, Atwood makes a 10 gallon heater that fits in a six gallon hat. How does it work? Simple. I mentioned a 180 T-Stat above, they use that one, heat the water to 180 (near boiling, in fact at this temp you coudl just pour it over yoru coffee grounds as that's in the range of many coffee makers) On the output of the water heater is a thermostatically controlled mixer valve, it adds cold water to bring it DOWN to 120,, Thus giving you about 10 gallons of 120 degree water from a six gallon hot water tank.
I you have this system (and from your description you just might) and that valve failed. (letting too much cold in)... Well.
Home was where I park it. but alas the.
2005 Damon Intruder 377 Alas declared a total loss
after a semi "nicked" it. Still have the radios
Kenwood TS-2000, ICOM ID-5100, ID-51A+2, ID-880 REF030C most times