I also have a 12 ft porta boat, and carry it on a 2955 2005 Montana. I just got it last year because I likes to bass fish and it works great! Have had it out numerous times and can not say anything bad about it. Bought mine new, and was pretty pricey so if you get it for free consider it a great bargain! It is not the ranger bass boat that I am use to but it works well with the 5th wheel. We carry it also under the Montana. With a little torch, welder and imagination I built the carrier. I made it out of lightweight steel, and put carpet on the floor. I bought both the trolling 45 lb thrust and a gas motor 5 horse Honda. I only used the gas once, seems the trolling is what is needed most of the time because I lake fish. I did take it out in a cove at the ocean to go clamming and that is when I used the gas motor.
It was a little hard to put together the first few times because it was stiff and new. There is a board that comes with the boat that holds it open while you unfold it and it is a must. Make sure you pick that up also. Can be easily made but without it you will be cussing up a storm trying to get the boat to stay open until you get the seats in.
It was a great investment for me. It is sturdy and safe. Just the concept of knowing it folds up everyone thinks it can do that in the water. It can't. The UPS guy who delivered ours said he saw a truck run over one and it never did anything to it.
I can try to take a few pics if you are interested send me a email to
wdanielsc@juno.com. I think you will be happy you made the trip. The only down fall is the new Montana's with the larger basements you can't put the carrier under it because it is to low to the ground. So if you have a 2008 or older Montana you should be ok no matter the model.
Good luck, keep me posted.
Bill