Hmmm NEAC website is ominously vague about public transport.
Says taxi from Coventry train station. Mentions Coventry bus station but no specific bus nos. or routes ...
http://Www.traveline.info is both a journey planner and has a tab to access the full bus timetables for every route in the country.
As far as I can tell, the direct bus service is the 539 from Coventry to Kenilworth via Baginton and the NEC, where it stops outside the main entrance. Four buses per day , takes just over half an hour.
Bus service 539
No mention of Kenilworth Station on the website
Are there no special/shuttle buses when an event is on?
There are hotels near Chesford Bridge about 3 miles from the show ground on regular Leamington Kenilworth Coventry bus routes but very limited service via the Showground and even is there was it could be a longish walk from the main road to where you would want to get to on the site
Not that I can see on their website - possibly only for exhibitors.
There's this one, although not a frequent service:
HI. The Coventry City Centre (Belgrade Plaza) Premier Inn is probably the nearest to Cov train station. I drive this route past the NAC to/from work and can honestly say I've never seen a bus on this route, but I could be wrong. I would say that your best bet is a taxi from Coventry as I would say it is nearer to Coventry than anywhere else.
I have found this on their website - https://www.naecstoneleigh.co.uk/Accommodation/Accommodation_Home.html .
Coventry train station is closer than Warwick Parkway, when travelling by train. https://www.naecstoneleigh.co.uk/Visitors/Getting_Here_1.html
If you have not already booked something, you could probably book the Premier Inn on the A45/Kenpas Highway, easy to find on a map... located behind a Miller & Carter restaurant. Then you could probably take a taxi to the showground which wouldnt cost too much.
If you would like a good recommendation for place to stay/eat/drink try The Oak in Baginton, The Old Mill in Baginton and maybe stay at the Dakota in baginton. That is very close to the NAEC/Showground.