04 E350 6.0 T-Stat problem?

[04 E350 Van]

Greetings...
I have a couple questions about my van w/ 235k miles that runs great... this was my work van till the company bought me a new freightliner sprinter van and I bought this one which I'm using to tow a 7X12 enclosed bike trailer.
This van doesn't get used much and I noticed the last 2 times I ran it my T-stat wasn't working and my engine temp never got above 150 degrees F.
I'm planning a road trip with the bikes in a couple weeks into the TN/NC mountains and I'm wondering if the t-stat is not working will this effect fuel mileage? Does the engine need to be operated at a 'ideal' temperature for everything to work effeciantly(sp) etc.
I bought a new T-stat and don't have a service manual... what is the best method to get at the T-stat... from the front or in the doghouse? I looked under the hood and holy crap I couldn't see the top of the block or where the T-stat is located.
Any help would be appreciated... Thanks in advance
Tom