Answer: You've already recognized that it really is a matter of personal preference and how each type of tenancy fits into your park's business model. There is certainly an advantage to having long-term tenants so that you have less turnover and you develop a good reputation as an option for residents who plan on staying awhile.
However, month-to-month tenants can only be evicted without cause on 30 days' written notice during the first year of the tenancy, and on 60 days' notice after the first year of tenancy. (But see special note below for Portland and Milwaukie landlords.) Additionally, state law now prohibits rent increases on monthly RV tenants during the first year of the tenancy and requires 90 days' written notice to raise the rent after the first year.
Conversely, week-to-week tenancies make it easier to evaluate a tenant in the short-term to ensure that he or she will work out in the long run. Weekly tenants can be evicted for no cause on 10 days' written notice and rent can be increased on 7 days' written notice. These are significant advantages when evaluating tenants as possible long-term residents.
The primary disadvantage in creating weekly tenants is the administrative burden. To create a week-to-week tenancy in Oregon, it must have all of the following characteristics: (1) occupancy charged on a weekly basis and payable no less frequently than every seven days; (2) a written rental agreement that defines the landlord's and the tenant's rights and responsibilities under Oregon law, and (3) no fees or security deposits, although the landlord may require the payment of an applicant screening charge.
In your situation, it might make sense to put new tenants on weekly agreements at least initially. If they seem to be working out, you can then offer them a new, monthly agreement later. MHCO Form 80 [Recreational Vehicle Space Rental Agreement] can be used for either weekly or monthly tenants. Just be sure to properly fill out the form and do not charge weekly tenants a security deposit or other fees (or you risk automatically making them monthly tenants right away).
Special Note Regarding RV Parks in Portland and Milwaukie: These two cities have enacted local ordinances that override state law and require a minimum of 90 days' written notice for no-cause evictions from the very beginning of a month-to-month tenancy. Landlords in these cities might want to seriously consider the week-to-week tenancy option for RV tenants since weekly tenancies are specifically exempt from this regulation in both cities.