I am planning to post separate answers for John and William, because there are several of each baptised at Okehampton in this period.  This answer will examine whether John's life post-baptism may shed any light on whether his parents have both been correctly named there.

In the 10 year period either side of 1800 there were four John Milman baptisms at Okehampton, and two more if that period is extended to 20 years.  I am not going to extend my search to the surrounding parishes for the moment.

 1. John Milman baptised 4 Jun 1791 at Okehampton to mother Grace
 2. John Milman baptised 11 May 1800 at Okehampton to parents William and Ebbett
 3. John Milman baptised 4 Aug 1805 at Okehampton to parents John and Martha
 4. John Milman baptised 12 Feb 1809 at Okehampton to parents Joseph and Elizabeth
 5. John Milman baptised 16 Aug 1811 at Okehampton to parents James and Elizabeth
 6. John Milman baptised 29 Oct 1815 at Okehampton to parents George and Susanna

The two that I am going to focus on first are the second and third Johns baptised 1800 and 1805 respectively.

On 26 Oct 1826 a John Milman married Joanna Lavis at Sourton and they appear to have had these children:

 - Mary was baptised on 2 Sep 1827 at Sourton to John (Labourer) and Joanna
 - Elizabeth was baptised on 26 Dec 1830 at Bridestowe to John (Labourer) and Joanna
 - John was baptised on 29 Oct 1833 at Bridestowe to John (Labourer) and Joanna

The family appears in the 1841 Census at Bridestowe:

 - John, aged 40, Ag Lab, born in Devon
 - Joanna, aged 35, born in Devon
 - Mary, aged 13, born in Devon
 - Elizabeth, aged 10, born in Devon

If this John was baptised at Okehampton (more evidence still needed), and if this John and Joanna had no other children it suggests that John named his eldest son John, suggesting that John's father may also have been named John, and there was a John Milman (married to Ebbot) baptising children at Okehampton around that time.  The evidence is still very tenuous but I think it suggests that the parents of the John Milman baptised 11 May 1800 at Okehampton were John Milman and Ebbot Martin.

The absence of the name Martha from the children in the Bridestowe family suggests that the girls Mary and Elizabeth may not have been granddaughters of John and Martha who had their son John baptised in 1805.

I am hoping to include more evidence in this answer to try and refute/support the suggestions that I have made so far.