Mick thats defo on the low side.....when I say I get 25-ish, thats driving like a looney. I've just got back from Wales and been doing 70+ or so on the A-roads and 80-90 on the motorway (indicated of course)...and that will see me getting 25 mpg as I do the journey every week and measure regularly.
Its worth measuring it over a few tankfuls for several reasons; weather can make a bit of a difference, petrol station pumps don't always give exactly the same amount of fuel and even fuel varies from one place to another, plus pump cutoff varies a bit as well - it all adds up.
If you find you are still getting 20 in mixed driving I'd start looking at the simple stuff first; air filter choked up and in need of change etc.. - and I would throughly recommend a free-flow air filter like the K&N (either a straight replacement for your existing filter at about £30, but never needs replacing - you just wash it in petrol and re-oil it with the special oil they supply, and even that is every 30-50k miles or something...the filter is good for more miles than your truck will ever do. Or £50-odd will buy you the induction kit - basically a conical filter stuck in the breeze with an adaptor pipe to replace your existing airbox (which sucks air in from the inner wing).
On both my trucks thats been good for at least +2mpg and a little bit more oomph.
Ignore anyone who tells you you'll drown the engine if you go in deep water - if you don't do that stuff it doesn't matter, and unless you had a snorkel, your electrics would drown before the filter started sucking water in.
But if the filter is clean and free-flowing you might have to get a bit more diagnostic - thats when I take the bloody thing to the garage cos I can't be bothered these days
but theres lots of v.clever folks on here with good technical skills to offer you a few more pointers.
But 25mpg minimum definitely your target, and more if you drive sensibly....