Welcome to the Jayco forums
Until someone chimes in with a similar TV/TT combination with some first hand insight, I hope the following information will be helpful in the mean time .....;
Your correct, the CCC of the X17Z is very limited and it won't take much cargo to max out the TT's GVWR (3,500lbs). The published CCC is only 675lbs, but it is based on the UVW weight. The UVW of 2,825lbs isn't the ship weight (look for yellow sticker on TT), so that will add 100lbs to 250lbs to the UVW (depends on Jayco model). The sum total weight of the battery, WDH, and any dealer options will have to be added to the ship weight as well, so IMO your guess of about 300lbs to 400lbs maximum cargo sounds about right.
The safest, and most accurate way to address your particular TV/TT loading question is to visit your local CAT scale. Basically you don't want to exceed the GVWR of your TV or TT...., the CAT scale will tell how to load your specific TV/TT combination and stay within all of the specified weight limits.
Visiting the CAT scale will provide you with a lot of useful weight information, plus piece of mind. The following thread link will guide you through the CAT scale weigh-in process: http://www.jaycoowners.com/showthread.php?t=3871
I apologize that I can't give you a definitive answer to your question, mainly because IMO your personal loading "style" will have a great influence on the actual TV/TT loaded weight outcome.