People (including myself, to some extent) seem to regard River Park Short Term High Yield (
RPHYX) as an
enhanced cash fund. That means that it is expected to have good (but not perfect) price stability, superior yield to a MMF, and a portfolio not constrained by
Rule 2a-7 (essentially 60 day/13 month maturity restrictions).
Bank loan funds don't seem to come close to this ideal. Just look at the huge price drop in 2008,
even for FFRHX. (Admittedly, several funds marketed as enhanced cash funds, notably Schwab Yield Plus, failed miserably also.) So I don't see bank loan funds serving as an alternative fund.
As to why the Loomis Sayles fund has a much higher yield than the Fidelity Fund, several factors pop out: LS
credit rating is somewhere between B and CCC (60% of fund rated B, nearly half the rest is lower), vs. BB for the Fidelity fund (higher than most bank loan funds); LS can hold fixed rate securities (longer effective duration, or equivalently, more interest rate risk); LS can use leverage.
Finally, note that the SEC yield between the two funds is smaller than the TTM (trailing twelve month) yield difference. SEC yield is yield that takes into consideration total return - if you buy a bond at a premium (above face value), then it gradually declines in price (NAV) over time. So the total returns are somewhat closer than you may think (though there's still quite a difference between the funds).
In the muni arena, BobC has suggested NEARX. I'm not quite as sanguine about this fund as he is, but I'm happy to acknowledge that it's worth a serious look. (You didn't say whether you were looking for a taxable or tax-sheltered account.)
As I've written before, I like FPA New Income (FPNIX) as a conservative bond fund, but it too is unlike
RPHYX. It is certainly not positioned as an enhanced cash fund.