You are actually dealing with two different factors. 1) volume, and 2) pressure. Filling the empty volume of a tube takes most of the effort, because you are filling empty space. Once you've gotten most of the volume filled, raising pressure is fairly easy. I don't trailer, so have put fair study into the subject, and talked to NRS, who educated me on this.
I deal with the volume problem with an old LVM 12v pump, that fills the tube, but leaves me at around 1#, which makes for a soggy row. I use a couple of different hand pumps to top-off and bring up my pressure. At 1#, my little K-100 will get me to 3# fairly quickly. I spent 6 months this winter waiting for an NRS 12v Bravo pump, which in theory will get to +13#s, only to discover that a high pressure pump produces amazingly low volume. Great for low volume/high pressure tubes like SUPs, tediously slow for high volume/low pressure tubes like a 14' SB. So, the best pump choice is highly variable, depends on what you want to,fill.