Do you know if your bike still has the charcoal canister still installed? If so there seems to be some thoughts that charcoal migration can start to plug the vent lines. If it is still installed, I would pull off your right hand side fairing. You should find just behind and below the fuel tank 2 vent lines each with a union coupling. 1 is for the tank cap water drain and the other is part of the charcoal canister tank vent system. I don't recall with the installation which line is for which vent/drain. My thought is to make a small colored mark on each line on each side of the coupling (i.e. white dot on one line both sides of the union and yellow dot on the other line both sides of its union). This way they don't accidentally get swapped when you split/disconnect them at the union. Alternatively, you can separate each line by itself and then reconnect before moving to the second line. The idea here is to see if this may relieve any negative pressure that might be holding the cap from opening. The cap drain line should just be ambient pressure as I believe that is an open drain for water to follow down to just behind the riders right hand foot peg. The tank vent should go back to the charcoal canister and if that line was plugged this MIGHT relieve the suction... if there is any. If the charcoal canister was removed form the bike previously then both of those 2 vent/drain lines shouldn't maintain any pressure as the are both at ambient pressure and again routed down to just behind the riders right hand foot peg.
The other thought would be to split the fuel supply and return to see if that would relieve negative pressure. If your fuel lines have quick disconnects, once each are separated, then you may need to carefully unseat the self sealing check valve in each half of the supply and return on the tank side. You might get some fuel coming out, but if there is a contained negative pressure in the tank, then I think you would get an initial air inflow into the tank and maybe not any fuel coming out.
I have an 1150RT and removed the charcoal canister a few years ago and have never had any tank venting issues. I did replace all my supply, return and vent lines also and have them color coded to reduce the likelihood of swapping lines around. Once you have the cap open you can check if the lines did get swapped previously by blowing low volume air through them, with the cap drain being the easiest to diagnose. The main thing is to first understand the current system (canister installed or not) and them move on from there. There have been cartoon illustrations of the system originally and if the canister has been removed. From that you can deduce each lines connection points and confirm all is correctly installed. Search the forum for other threads on charcoal canister removal and you should find it.
I am not really sure if a negative pressure can be created to the extent to prevent the tank cap from being opened thought. Other more experienced forum contributors may have better first hand experience and should chime in shortly. Hopefully I haven't given you any bad info or sent you on a wild goose chase.
Thanks and hope you get this figured out relatively simply and quickly.