You say your DTTV or box will not get signal with your 315, but there's more to it than that.
For a start where are you trying to get a digital signal. If you are at home, 30 miles from the serving transmitter, and everyone around you has stonking great multibeam aerials the the 315 will definitely not work.
If on the other hand you live 10-15 miles away with a relatively unobstructed path to the transmitter then it should work, at least partially.
Some elementary checks are in order: is the power supply box switched on (red LED lit?); is the attenuator switched out?; are the plugs in the right connectors?
If the answer to all is yes then start taking apart the plugs on the cables built in to the van, i.e. that on the end of the cable from the aerial and that going away from the aerila power box - assuming they are not molded on. Check that there are no sprigs of wire - single strands - shorting the screen to the centre core. If the main connection is via the fixed outlet socket take that apart as well and make the same checks. I found shorts on two out of four connections - and thereafter the 315 worked - shall we say - somewhat better?