Am I entitled to a meal or subsistence allowance for fieldwork?

According to the Working Hours Act, you may not work for more than 5 consecutive hours without a break. The break is unpaid and must be scheduled as lunch, for example. From your description, I understand that your work requires a high degree of flexibility and that you are out most of the working day. However, this does not detract from the employer's obligation to organize the work so that the working environment for the employees is reasonable and that you have the opportunity for breaks at work.
Subsistence allowance/meal allowance is generally only relevant for business trips where the employee has increased living costs due to the trip. If your employer has a collective agreement and/or travel regulations, there may be rules governing the right to compensation. You can read more here: Traktamente - privat | Skatteverket External link.
In daily work, there is generally no obligation for the employer to pay for lunch costs, but they must ensure that you have the opportunity to take a reasonable lunch break.