After running several portable operations using different antennas and I have had the best results using two 20 mtr ham sticks as a dipole. I use a 22 ft fiberglass flag pole I purchased several years ago at an RV show. The vendor said I must be a ham radio operator since they are the only people that buy the 22 ft version. I home brewed a mount for the ham sticks and it has worked pretty well. The main advantage is you don't need any other supports, which if you are staying in a timeshare or other rental can be a plus. When we lived in the RV there were several places we stayed that would not allow any ropes in the trees or any thing driven in the ground, so the pole was necessary to get on the air. I use one of the ham sticks for my mobile operations. I am not sure a 40 mtr hams stick would resonate on 15. I have one and I may try it to see if it works. I have also found that trying to load them on other bands using a tuner, although the tuner could find a solution, they did not work well and caused some RFI.
I have used an end feed antenna and found that while it works I did not get very good results with it. When I have time I am going to see if I can work out a better wire length. I have also use a 23 foot wire with my AH-4 auto antenna tuner, which gives me all band coverage with only one support. I do use a counterpoise with it. Worked ok, but caused one of the arc flash circuit breakers at our friends house to trip and also caused RFI to the recliner in the family room, it would extend when I transmitter, hi hi.
As a couple of others have suggested I am in the process of building a add on dipole that will start at 17 mtrs and allow extra wire segments to be added to cover 20, 30 and 40.
The other thing I liked about the fiberglass pole, is that at night I could collapse it and the antenna virtually disappeared. I would only extend it when I wanted to operate.
These are my thoughts and I am still working on various solutions for portable ops, but if you don't try something you won't discover what works.
Dick - KC3EF