If the coax is double screened (braid and foil) and has you say its got a solid diwlectic insulator, the coax is likely to be WF100, TX100 or PF100. Although it is possible to get CT100 with a solid dialectic. Can you remember where you purchased the coax from and the manufacturer.
Have you tested the coax for dead shorts with it unconnected from the booster and aerial.
Have yo examined any external coax for damage of the outer sheaf.
If you aerial is aligned correctly, the problem is likely to be the booster or Aerial diode/balun failure. However because the coax is the cheapes’t thing to replace, that is what I’d be doing first. partly for the simple reason your multimeter won’t pick faults or than a dead short.
I’m not a fan of these fancy expensive looking tri boom aerials, much prefering a log periodic or a yagi aerial. A log aerial hasn’t got the gain or sensitivity of a Yagi aerial but, they are directional and tend to provide a clean signal without noise unwanted noise or signal. But unless your aerial is at fault I wouldnt bother.
If you are friendly with your neighbouring carvans providing you have a long enough lengh of coax and a couple of adaptors you could ask them if you could borrow the signal from their aerial. Firstly directly to the TV and then inputed into the booster. This would then limit the fault to the booster or your aerial. However it won’t highlight faulty coax, so were back to replacing this first being the cheapest option 🙂