Not sure if you're still considering it but, for future reference, I can recommend the Limes campsite in Great Steeping near Spilsby. We were there at the weekend in our baby caravan, but without bikes. It's a small site, pretty isolated, so very quiet and with basic but good facilities. The site manager/owner is extremely friendly and helpful; when I trotted out my reasons for not cycling in the area - as mentioned by others above - he corrected me and said that the wolds to the north of Spilsby had lots of quiet back roads and offer some good cycling.
(Edit: I seem to be getting my Fens mixed up with my Wolds. I blame inattention during Geography classes, compounded by old age.)