I'm afraid that you are working within a very constrained memory environment on the iPad1 running 5.1.1. Disabling multitasking via jailbreaking is definately not the right route to explore IMHO. That makes the device consume more, not less memory, even running in a single app scenario.
With 256MB of RAM and IOS 5.1.1 to run, things are extremely tight. If you need Safari to perform optimally, especially on large / complex websites, then the best thing you can do is minimise your RAM footprint by removing as many tweaks as possible.
Simply trying to force Safari to stay in RAM is exactly the thing which will cause it to crash when IOS decides it has run out of memory.
Sorry it's not a more positive solution, but going back to basics may be the thing which helps you most, if your primary usage of the device is browsing in Safari.
If you have saved SHSH Blobs you could also consider going back to a 4.x IOS, such as IOS 4.3.3, which has a smaller RAM footprint (although in 256MB, the margin is not massive). The downside of this being the loss of features such as iCloud and incompatability with a growing number of apps, etc. We have guides and tutorials on this in our Hacking Section if it is of interest.
Hope that helps, and good luck!