If you are paid an amount per night stop, your allowances are for subsitence down route. As such, they should neither be taxable nor should they be accountable as salary in any way. They are an expenses payment. If you are paid an hourly rate then it can be regarded as a productivity payment, which is taxable and may be regarded as variable salary, though if you are expected to pay for your meals from this then an element of it should be free of tax - and thus can not be regarded as salary.
You - or your union - need to talk to a tax advisor to sort out what your variable payments count as, and whether they should be taxed or can be counted as salary for the purposes of a mortgage or other loan. You might not like what you find out! As for leaving the line to become a manager, it's the same for travelling salesmen and similar jobs all over the place. As your expenses on behalf of the company diminish, so do the payments to cover them.
Scroggs