A probable marriage from the Berkshire IGI (Familysearch):
12 Dec 1797 William MILLS and Letitia LODER at Hagbourne, Berks
The IGI has no baptisms to this couple in Berkshire and, as I note that the first child baptised to William and Letitia in Great Marlow was in 1798, this looks very likely.
One reason why I think this marriage could be the right one is that there is what seems to be an illegitimate child baptised to an Elizabeth LODER in Hagbourne in 1805. This child was given the middle christian names of Owen Weatherhead. The WEATHERED family were wealthy brewers in Great Marlow and I suspect that one of them may have been this child's father. Perhaps Elizabeth LODER was a sister of Letitia.
Another reason for this marriage being the right one is that the surname ELDERFIELD occurs in and around Hagbourne whereas it is fairly uncommon in Buckinghamshire.
The database search you had from Bucks FHS would have covered the whole of Bucks.
With regard to the 1841 and 1851 census it is quite probable that you won't find either William and Letitia as they may well have died before that time. I think that William may have died in Great Marlow in 1813 as there is a burial for a William MILLS, aged 36, on 23 Nov 1813. There are no other MILLS burials in Great Marlow between 1813 and 1866. If this is 'your' William he could be the son of Richard and Mary MILLS baptised at Great Marlow on 22 Dec 1776. Following William's death Letitia may have returned to her home county of Berkshire with her family and this may be why Priscilla married in Reading.
I hope this is helpful.
Orpheus
PS: I note that William and Letitia had ten children - Mary (1798), Sarah (1800), Harriot (1802), William (1804), Richard (1805), Joseph (1807), Letitia (1808), Maria (1810), Eliza (1812) and Priscilla (1813)