Low GPS accuracy

Ok I have a TZ14 and a TZ9 secondary unit. along with radar/heading sensor/and sonar.

last 2 times I have been out I have noticed a large discrepancy between the cog and heading values displayed on screen. around 15-20 degrees. When I pull up the sky menu for the GPS none of the sats have a signal strength over 25%. No cloud cover and on open water with 360 degree view of sky to horizon (minus the radar dome of course) The Hdop stays steady around 1.

I feel like I should be getting better signal strength than this. I checked the antenna and it is clean and secure. I had all other electronics such a AIS and VHF and radar off just to eliminate interference possibilities.

Any thoughts or ideas? Does this sound right?

P.S. I am running the latest SW 3.01
There are many variables. You REALLY want to make sure it is mounted outside the recommended withdraw areas of your radar/VHF/SSB/AIS/Wi-Fi boosters etc. For example a GPS antenna can get damaged if you continually allow a radar to blast it with RF when the radar is being used. It normally damages the front end amp of the GPS and the signal strength will get worse and worse as the amp gets more and more damaged. The amp will still show damage even if you turn off the other devices. The damage has already been done to the amp. A better test would be to compare the level when you run everything (including radar) to when you are not. If the problem is worse when you are running other items, then it has most likely is getting blasted by something. You can also see similar issues if you allow water to get into the GPS head. It will start having problems and become more intermittent until it breaks entirely.

This morning my GP330B was getting about .8 Hdop with most SATs showing greater than 25%. yours isn't too bad but sounds a bit low. You might want to keep an eye it. It is normal to see differences between COG and Heading especially if you are turning or just completed a turn. You can also see differences if you are taking cross currents. COG/SOG isn't valid data unless the boat is moving 3knots or more. I recommend running the boat on a straight course without cross currents and compare the heading, COG, and your steering compass. They should all three be within a degree or two. It might just as well be your heading being pulled of by something on the boat and not related to your GPS. Your TZtouch has WAAS setting that you can turn on for your GP330B to get a better fix. It also has a damping (smoothing) setting for your SOG/SOG that you might also want to check. Setting your damping too high will not allow the data to react as quickly and setting it too low will cause the data to jump around too much.
I will try to take the boat out Friday if weather permits and check on it. The antenna is as far away from the radar as possible, but is forced close proximity to VHF/AIS antenna. (boat design...)

The COG is usually pretty close to compass heading but the heading is sometimes off pretty good. I read in the heading sensor manual that it can be (re)calibrated. Think this will help any? It is a PG700 heading sensor.

Also in the menu of the TZ14 I see offset values for heading input. If the heading sensor is consistently off a certain degrees would this be a viable option.
VHF/AIS should be 1 meter (3 feet) away from your GPS antenna, minimum.

It is always a good idea to calibrate your heading compass. If you find the heading being off after being setup properly, take a look at the status light. If it is flashing, it has an error. (most likely from big metal, electrical, speakers, or unsuitable mounting location)

I would start by removing any offset you have placed in the MFD.
If after calibration of the compass your heading is still off, you should conduct a heading adjustment (pg 11 of PG700 manual) to make it reads perfectly. This way the heading data is exact for ALL items on your NMEA 2000 network that is reading your compass. If later you find heading off, check the status light.