You are probably referring to the Royal Cornwall Militia. This unit was deployed to Devon in March 1797, as part of the coastal defence against an anticipated French invasion. In all likelihood, this would be why Francis Green was in the parish of Totnes the next year.
6th March: The Cornish Militia came into Dover to be quartered in Town during the Election at Canterbury
- Diarty of Thomas Pattenden (1748-1819), Resident of Dover
(The previous election for Canterbury, in 1796, was voided for bribery on 2 March 1797, and a by-election was held. MP Samuel Elias Sawbridge won again on 10 March only to be removed later in May).
Such deployments were a typical experience for all British Militia during the Napoleonic Wars. In this turbulent era, the regular Militia was mobilised and transformed into a reserve force for the Army. Most regiments rotated through deployments outside of their home counties, while volunteer and local militia units were formed to replace their traditional role in home defence.
During its long wartime embodiment, nearly all the militia regiments were stationed away from their home counties, losing their local ties, and came to be treated as a source of recruits for the army.
- Gee, Austin. The British Volunteer Movement, 1794-1814. Oxford University Press, 2003.
In fact, such rotations throughout the British Isles would come to be seen as a beneficial exercise in nation building in this early age of nation-states. Its effectiveness in reality is another matter, though.
It was, however, widely believed that the militia could act as a vehicle for national integration. When a bill to authorize the service of the Irish militia in England was introduced in parliament in 1811, its proponents stressed the beneficial moral and political effects of such an exchange: 'new connections and friendships would be formed, not confined to one class or degree but extending generally through both nations'.
- Kennedy, Catriona. Narratives of the Revolutionary and Napoleonic Wars: Military and Civilian Experience in Britain and Ireland. Palgrave Macmillan, 2013.
Note: Modern British Militia were authorised by the Militia Act of 1757, which organised regiments on per-county basis. So a "Cornish Militia" wouldn't have been the militia of a Duchy of Cornwall, which was and is really a collection of manors moreso than an actual geopolitical unit. Though, the Cornwall Militia did become the Duke of Cornwall's Rangers.