I used the PU18 that was suggested by wizard on his website,we used that on the inside, very impressed with it. I had also bought Sikaflex for £10 from Go Outdoors, so used that on the outside. Probably should have used the PU 18 for both.
I use mine most weekends with inner panels missing lol, as its kept at a friends 12 miles away so dont always get alot of time to work on it....have even been known to take supplies with me and work on it in the evenings at agility shows !!!
Most sealant on older caravans is oil based so should dissolve with white spirit or use cellulose thinners very carefully if nothing else will shift it. If the floor is dry you can patch it with IsoponP40 filler, rather than mess about cutting sections out & replacing.