Whirlpool Gas Water Heater Woes

Greetings,
I purchased a 40 gallon Whirlpool gas water heater (including installation) from Lowe's in December of 2003 to replace a 30 gallon water heater. Before, the 30 gallon water heater was sufficient to fill our oversized bathtub, but we wanted to increase the size anyway.
In a nutshell, the new water heater is unable to fill our bathtub with hot water as before, despite being 10 gallons larger. We now get about 2-3 inches of hot water.
After careful measurement using a 2 gallon bucket, we found out that we get approximately 22 gallons of water after a 12 hours of recovery time. After 20 gallons, the water becomes worse than lukewarm. For now, we are OK with showers (I like days, she likes nights), but we would like to be able to take a bath sometime over the next few years.
Upon the plumber's recommendation, we had Whirlpool send us a new thermostat. After installation, the performance is exactly the same.
Whirlpool said that we should be able to get 27 gallons (70%) of hot water, and that we haven't done enough troubleshooting on the heater. Whirlpool indicated that we would need to check the mixing valves (our bathtub has separate valves!), implement low-flow shower heads (we already have them, and care about the bathtub), or have them replace the thermostat (a second time). They indicated that they would not replace the water heater unless I could prove that it was damaged in some way.
I'd like to check the dip tube, but I believe it requires the plumber to come back. Is it possible for me to check the dip tube myself? When I asked Whirlpool about it, they made it sound like it's as easy as replacing an air filter...
Am I expecting too much from this 3 month old water heater? Any other thoughts?
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mrthejab wrote:

Its not your job to diagnose the heater, its Lowes. Tell them to make it work or to replace it. Be firm and dont take no for an answer, you paid them to install a "hot water heater" it has ratings, if it doesnt measure up then its no good. I'm sure you didnt intend to purchase a cold water tank. If you dont get satisfaction, see the store manager, if that doesnt do it get a lawyer. Eric
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