I deal with several queries every year about towing caravans to the site - I'll be doing better maps shortly. However, for now, there are 5 roads into Sidmouth from the A3052 coast road.
Starting with the one nearest the east (from Seaton and Lyme Regis direction):
Fortescue Road - this is half way down the steep hill (Trow Hill) into Sidford. Avoid it.
Next is the major traffic light junction at Sidford Cross (junction of A375 and A3052). This is OK for buses and HGV into town but turn right at the mini island into All Saints Road before you get to Sidmouth High St. See Festival maps. Turn right again at Woodlands Hotel (another mini-island at top end of All Saints Road) and this takes you to the campsite easily - it will be on your right.
Next is the turning by Waitrose - Stowford Rise - no real point in taking this one.
Next is the Woolbrook Road - ditto as above.
Some distance further west (towards Exeter) is the A3176 at the BOWD - a big pub/restaurant on the right.
BEFORE you get to this pub travelling west (or just after it travelling east), turn onto A3176 to Sidmouth. Narrow country road but takes you to festival campsite which will be on your left. This junction is the FIRST one to Sidmouth if travelling east from Exeter direction along the A3052. It is also the one signed with FolkWeek signs.
Don't get stressed if you get into Sidmouth with a caravan - it is easy. HGVs do it all the time. Just stick to the main roads!!!
WHATEVER you do do NOT try to get to the official festival CAR PARK towing a caravan. Parts of your caravan might make it, other parts might not...... And don't overshoot the festival campsite entrance, it is some distance before you can turn around. And avoid Ice House Lane. It is a shortcut for people and small cars only.