RV Repair: water heater, holiday rambler alumalite, consistent problem

Question
Have a holiday rambler alumalite (80's vintage) The water heater heats, hot water runs from tank thru all faucets fine for about 1 1/2 gallons MAX.  That is all we get until the heater heats water up again.  This is a consistent problem. I tried to make certain that the tank had no air in it. The previous owner swears that it produced h/w with out trouble before. Any suggestions would be most appreciated.

Answer
HI DEBRA
I CAN THINK OF TWO POSSIBILITIES...FIRST, IT MAY BE THE TANK HAS ACCUMULATED A FAIR AMOUNT OF SOLIDS (MINERALS) FROM THE WATER AND HAS NEVER BEEN FLUSHED OUT. THIS WILL CAUSE REDUCED CAPACITY OF THE TANK DUE TO THE ENTRAINED DEPOSITS.
THE OTHER IS, IF THERES A "BY-PASS" INSTALLED ON THE WATER HEATER, IT MAY BE EITHER OPEN OR LEAKING BY, ALLOWING COLD WATER TO MIX WITH YOUR HOT AND REDUCING THE AMOUNT YOU GET OUT.

THOM