Welcome to this wonderfull forum ! Out of curiosity where are you from and what do you know of this area ?. I ask as although Westbury On Severn is still quite a small village, up until 1937 it was also the name of the Registration District which incorporated a much larger area of South West Gloucestershire including the FoDean. Hence she may have lived anywhere in this large area, so anywhere within the huge database of Parish Records on this site.
If you search this site's Parish Records there are numerous Mountjoys & Thomas's.

eg quick search for all the Mountjoys on the database between 1740 - 1780, say, gives 28 records. 26 of these are at Littledean, a village on the edge of the Forest and only a few miles out of Westbury on the main road. Most appear to be the same family including Elizabeth Baptized Christmas Day 1768, her parents Anthony & Elizabeth and several siblings. Hopefully these names match your findings ! If so you have extensive records to peruse and hopefully enjoy !. In this respect I consider you lucky that your family's records go so far back, perhaps as Littledean is one of the area's oldest villages by far with a Parish Church dating to the 12th Century !. (This site lists 95 Mountjoy PRs from Littledean alone, earliest 1711 and 42 Thomas PRs, earliest 1702).
These further Records include her Marriage to John Thomas at Littledean Chapel, and her Burial. Despite there being Burial Records of 141 Elizabeth Thomas's on this site, plus probably many more with name variations (Eliza etc), there are only 5 between 1837 - 1839.

Yours is probably one of two recorded as buried in 1837, the most likely being

Record_ID: 236046
Entry_Number:
Year: 1837
Month: Feb
Day: 24
Surname: THOMAS
Forenames: Elizabeth
Residence:
Age_at_death: 64
Officiating_Minister:
Event: Burial
Cause_of_death:
Memoranda: wid[ow] of John
Notes:
Register_Reference: RG4/766
Page_No:
Parish_Chapel: Littledean Independent
Soundex: T520

A quick Google shows apart from the main Parish CoE Church which does have a Cemetery (see Bigland Transcripts on this site, sorry but I can't access it today) there's also the United Reform Chapel, which I presume may be "yours" ?

http://www.geograph.org.uk/photo/741237

The photo shows some Graves so you may be lucky.

For info see
http://www.british-genealogy.com/local-studies/littledean-village-gloucestershire.html
This excellent site also refers to the nearby hamlet of Greenbottom, and it's Baptist Chapel. It also contains 1851 Census extracts, so well worth a look.
I'm sorry but my knowledge of the various local religions is poor, no doubt other's wiser than me will know exactly which Chapel is "yours" !
Sincerly hope this helps, please take the time to study & enjoy the Parish Records here.

Finally;
In 1870-72, John Marius Wilson's Imperial Gazetteer of England and Wales described Littledean like this:

"DEAN (Little), a village and a parish in Westbury-on-Severn district, Gloucester. The village stands on the east verge of Dean Forest, near the South Wales railway, 2 miles NW of Newnham; has a post office under Newnham, a curious ancient cross, and fairs on Whit-Monday and 26 Nov.; and was once a market-town. The parish comprises 510 acres. Real property, £2, 320. Pop., 887. Houses, 165. The property is much subdivided. Coal and iron ore are worked. The living is a rectory in the diocese of Gloucester and Bristol. Value, £200.* Patrons, the Corporation of Gloucester. The church is later English. There are an Independent chapel, a national school, and charities £25."