Answer: It is now statewide law that rents cannot be raised at all during the first year of a month-to-month tenancy. After the first year of the tenancy, you are required to give written notice to the tenant at least 90 days prior to the effective date of the rent increase.
The written rent increase notice to tenants must state (1) the amount of the rent increase; (2) the amount of the new rent; and, (3) the date on which the increase becomes effective. If the notice is mailed to the tenants, you must add at least three additional days to the notice period (not counting the date mailed) to allow for mailing. The notice can also be hand-delivered to the tenants, but this means putting it directly in the tenant's hand - posting the notice is legally ineffective.
If you happen to have any fixed-term tenancies, be aware that rent can only be raised for those tenants in accordance with whatever the rental agreement says concerning rent increases. Week-to-week tenants can have their rent raised with at least seven days' written notice prior to the effective date of the increase. The same rules apply regarding the information that must be in the notice (amount, new rent, effective date), and regarding additional mailing time for the notice.
There is currently no limit on the amount of a rent increase, although there are rent control measures under consideration in the Oregon legislature. With that in mind, beware that any rent increase that you issue before the legislature adjourns might be subject to new laws. Check in on the MHCO website to keep apprised of any legislative changes.
Special rules also apply within the Portland city limits. By ordinance, a rent increase of 10% or more within a 12-month period triggers the tenant's right to terminate the tenancy on 14 days' written notice and potentially receive a "relocation assistance" payment from the landlord. These payments are based on the size of the rental unit and start at $2,900. However, it is questionable whether the ordinance as written applies to RV tenancies. If you intend to issue a rent increase of 10% or more within the City of Portland, you should first consult with a knowledgeable attorney to assess the risk of being subject to relocation payments.