Commercial Double Trailers are regulated in Pennsylvania.
RV's are NOT ALLOWED to triple tow in Pennsylvania.
Even if your medium duty truck is technically a tractor, you will have to comply with the commercial regulations in Pennsylvania.
Pennsylvania Vehicle Code - Title 75:
§ 4904. Limits on number of towed vehicles.
(a) General rule.--No motor vehicle shall be operated upon a highway towing more than one other vehicle except as otherwise provided in this section.
§ 4908. Operation of certain combinations on interstate and certain other highways.
(a) General rule.--Combinations authorized by section 4904(e) (relating to limits on number of towed vehicles) to have two trailers may be driven only on the types of highways and under the limitations set forth below:
(1) On the designated national network consisting of all interstate highways and portions of Federal aid primary highways having at least a 48-foot-wide roadway or two 24-foot-wide roadways and designated by the department as capable of safely accommodating such vehicles.
(2) Between the designated national network and a terminal or a facility for food, fuel, repair or rest having an entrance within the access limitation prescribed by Federal Highway Administration regulation of the nearest ramp or intersection, but only on highways having lanes at least ten feet wide.
(3) On highways marked with traffic route signs having travel lanes at least ten feet in width unless prohibited by the department on State highways or the municipality on local highways based on safety reasons and marked with signs prohibiting such vehicles.
(4) Between the highways authorized under paragraph (3) and a terminal or facility for food, fuel, repair or rest having an entrance within one-half road mile of the nearest ramp or intersection, but only on highways having lanes at least ten feet wide.
(5) Approval of a highway other than as designated under paragraphs (1) through (4) shall be obtained from the:
(i) City in the case of a highway in a city.
(ii) Department in the case of a State highway not in a city, except that the department will, upon request, delegate authority to approve routes under this subsection to a municipality which has been delegated authority to issue permits under section 420 of the act of June 1, 1945 (P.L.1242, No.428), known as the State Highway Law.
(iii) Municipality in the case of a local highway not in a city.